This is very long, but hopefully interesting. Again wall of text ahead:
At least in the circles I run in, Mabar seems like a failure. Between a grind system that few enjoy (collecting Motes from random spawns and camping and kill-stealing) and a raid-style Dragon that grouped random pugs together when you must co-op via general chat and one that seemed to cause lag in the majority or players both in the raid and throughout the game -- I feel this event could have been something that most players would have enjoyed.
The following is how I would have the season event:
I would have handled the seasonal event very differently. I would have made a true seasonal raid where you grab 11 friends and face the hoards of undead. Each section of the raid would become progressively harder and would allow you to upgrade your equipment to the next tier. This raid would not have a lock-out or be restricted to three days. Much like Tempest's Spine, players could freely reenter as often as they enjoyed, this is supposed to be fun after all. The raid would be ~ 60 minutes.
Much of the following ideas are not unique, rather they are pulled from other parts of the game which means that the code for the types of actions are already in the game. the concept would be to make this event a fun quest rather than a grind and lag fest.
Stage 1: Knocking on Death's Door. Suitable for a raid group of level 8 players.
Stage 2: The Invulnerable Guardian. Suitable for a raid group of level 12 players.
Stage 3: The Vampire Lord. Suitable for a raid group of level 16 players.
Stage 4: The Demilich. Suitable for a raid group of level 20 players.
Stage 5: The Shade Dragon. Suitable for a raid group of level 24 players.
After each stage a shrine will appear.
Each of these stages correlates to upgrading the Mabar equipment. For example Stage 1: After players finish this section they "speak" with a chest and receive a full end reward of BtA items. This list includes ALL the level 8 and below items. Players select one reward as they would from a 20th completion of a raid. This would "finish" the raid and further progress would be "optionals". This concept is similar to the Litany flagging mechanism where after speaking with the dragon the four chambers are "optional". Players can only choose one of these items each run, but they are free to finish the quest here, reenter and grab a different end reward. The items are BtA!
What this means is there is no grind to get the item you want. Complete the quest, get the loot. Each Stage after the first is an optional; after completing this optional players may upgrade the base item to the next tier (level 8>12>16>20>24 in that order.) This concept again is pulled from the necro series a la the trinket upgrades on each alter of necro 1,2,3.
Stages elaborated:
Stage 1: Knocking on Death's Door. Suitable for a raid group of level 8 players.
This stage would introduce an NPC guide similar to Cellimas from Korthos. This guide would not have much impact on the raid apart from being a voice actor narrating what is happening and giving players some hints on what to do. We will call this guide "the guide" in this post.
The raid will begin in Delera's Graveyard and undead will spawn in the graveyard, however they will have zero impact on the raid or the upgrade process. They are merely for ambiance. They can be killed and don't spawn in large numbers, nor do they drop mats. There are no mats needed for upgrading during this event. A large crack will appear in the ground behind the three Tombs of the Delera series which will be the entrance to the raid with the guide in front of the crack. The guide will bestow the raid and explain that the plane of night has broken forth and players need to assist the Silver Flame. The raid will not have difficulties except for Elite. (This is just a catch-all in case something screwy happens with BB, this action might not be needed, though the concept is each stage is more difficult rather that having a level 24 stage one)
After accepting the raid and entering the instance (which will not be randomly played players, but 12 players that formed a raid like any other) players will be given some background periodically from the guide. This is simply again for ambiance. The guide explains that there is a holy relic contained within these crypts and if the players can help him acquire it he can hold back the hoards of undead soon to breach. Players will continue through the raid with minimal fighting until they reach a massive door guarding a silver flame sanctuary. (this style of combat is similar to the amount of combat in the Lord of Blades Raid before the boss. Just enough to get your blades wet but not much of a contest for prepared adventurers. ) This is not a long path.
The guide explains he should be able to open the door to the sanctuary, but cannot. At this point the raid actually begins and undead hoards of level 8 appropriate will begin flooding the chamber. The objective is to bash down the door that has massive HP while battling off the undead spawn.
This door will not be much of a fight for level 24 players. The group will need to prevent the massive hoard (and I mean massive) from overwhelming the door smashers. After the door is smashed the guide will enter the sanctuary, and grab the relic which will drive back the undead. After some quick dialogue the raid will "end". Player will be awarded with XP if they are at level otherwise regular XP penalties will be experienced. A chest will be in this sanctuary, that when activated will give players the full list of level 8 and below (level 4) Mabar items. (Cloak, Gloves, Wands, etc.) Players may freely choose one item which is BtA and either finish the raid or continue to upgrade them.
Stage 2: The Invulnerable Guardian. Suitable for a raid group of level 12 players.
After collecting their rewards the guide will explain that players should leave while the hoards have been driven back. If players continue through the sanctuary, the guide will reluctantly follow them. the guardian will place the holy relic back on its pedestal to prevent the hoards from following players and from entering the graveyard, but will warn the players that without its light players will face ancient horrors sealed within the Tomb. Further down the raid again with minimal fighting (now appropriate for level 12 players) players will see a short cut scene with a massively chained skeletal Giant which breaks its chains.
The players will now be in combat. While the Giant -can- technically be damaged in a usual fashion, but his HP/DR will be a huge deterrent. Level 24 players might be able to simply steamroll him, however the objective is to entrap the giant rather than kill him. The next idea was taken directly from the Titan Raid: as players fight the Giant seven columns around the room will be damaged by his massive blows.
The pillars cannot be damaged by players - each time a column is hit by the giant guardian, rubble will fall from the ceiling encasing three random players (identical to the encasement of the abbot except that the rubble can be broken away via standard damage, no boulders are needed.) After a pillar has been attacked three times by the guardian Giant, it will collapse encasing six players. After all seven columns have been downed, the ceiling will collapse around the Giant guardian "killing" him. The giant's blows will deal massive damage and fighting him directly is not designed, though as mentioned can be done if players are suitably powerful. (this concept is for level 24 players wishing to "speed things up".
After the giant has been encased, an alter will be opened that players can upgrade their items. There are no mats needed, only the base item. If players have several items, they may all be upgraded here. A short cutscene with an undead beholder will pop -up and the guide will tell players it hasn't sensed them yet, if they leave now they can avoid this fight. The rubble encasing the giant guarding will form a stairway that players can access the beholder. this will be a warning stating extreme challenge. It will be suitable for level 16 -well geared- players. The beholder will be very tough and similar to a strengthened version of the Doomsphere. (It must be strengthened as there are 12 players not 6.) There is no reward for defeating this beholder, rather its a lead-in to the next Stage.
Stage 3: The Vampire Lord. Suitable for a raid group of level 16 players.
More minimal fighting leads the players to the Vampire Lord's chamber. This fight the guide will explain that this was a very powerful silver flame cleric that chose immortality and she was sealed in these caves. The vampire will have all the standard immunities vampires enjoy in addition to Major Boon of Undeath: players striking the Vampire Lord will cause a negative energy burst centered on the vampire. This effect will damage players not Deathwarded and heal the Vampire each hit. This makes melee ineffective (though possible, again high level players can mostly ignore the healing it will do and power through him) Additionally he will have a Death Aura active at all times again healing her and damaging players. This fight will be timed, as it starts the vampire will release a toxin designed to transform the players into undead. this effect will cause constitution damage each minute that cannot be restored with restoration. If the Vampire is not defeated in ten minutes all the players will die. If the vampire is defeated within the time frame the poison will stop and once players rest the effect will be removed. The vampire is a level 24 Undead cleric will suitable spells / effects.
As usual, after her defeat, an alter will appear with level 16 upgrade.
Stage 4: The Demilich. Suitable for a raid group of level 20 players.
If players wish to continue the guide becomes increasingly nervous saying the last guardian chose to be here rather than being bound. A seal-door will ask players to continue with an extreme challenge warning.
More minimal fighting for ambiance will lead the players to an underground chapel. A demilich will begin a conversation telling players he is from a time long past where archmagi had limitless power and offer players one last chance to leave. If players continue, the demilich will invoke a wish: summoning the Invulnerable guardian, the Vampire Lord, and the Beholder to his chamber.
This fight will be intense for level 20s and it's timed. The Vampire Lord's Negative energy burst and Death Aura will heal the other 4. The beholder will make casting annoying while the invulnerable guardian is still hugely resistant to damage of all types. On top of these issues there is now a demilich in the fray. Immune to spells 6th and below (no disintegrate on this one...) It has its own Death Aura healing the others and casts at level 30 standard Wizard spells. If this wasn't enough, it also has access to Energy Burst. Each time the energy Burst is off timer the guide will tell players to Look out (similar to the strategy used on Nurse Ratchet in Sane asylum) Pillars around the room are intended for players to hide behind. This energy burst will almost certainly one shot any caught in its blast. These pillars around the chapel become damaged each energy burst and encase players as they did during Stage one and after 10 minutes all the pillars crumble encasing all the players effectively failing the raid. (The guardians remaining will have free reign to decimate the players, though the Skeletal guardian might manage to free a player inadvertently. This action might allow to raid to be saved if he frees the right person. )
If, by chance this stage is completed, an alter will appear where items can be upgraded to level 20. With the Demilich's death the final seal will open to the chamber of the Shade Dragon.
Stage 5: The Shade Dragon. Suitable for a raid group of level 24 players.
This stage will use the current set up. For players must man the switches to shine light when the dragon appears to damage it. After enough damage is done the dragon will drop. this fight be be incredibly tough as four of the players will be manning a switch and guarding themselves while the remaining eight are needed to either guard the switch players of damage the dragon.
Defeat will add the final alter and end the raid.
(the last few stages were rushed I needed to get back to work and this is already a long...... suggestion. )
I hope this was at least a fun read, as I kind of doubt any of this will be added.