Consider this not suggestions but a hypothetical game. If you could totally recreate the game of DDO from the bottom up - what changes would you make? Playable classes, races, how the XP is handled, the way loot is dropped - whatever comes to mind. Imagine wiping the slate clean, how would you redesign the game?
(Bonus points - keep in mind that it is a business. How would you handle the f2p vs p2p? What perks would you charge for vs giving away for free to balance out the fun while making a profit?)
My own idea's - Most of the changes I'd make would be following a mix of second and third edition rules to find more of a balance in characters/toons.
The setting - The primary setting would be a free to play area based on Greyhawk (or a forgotten realms type setting). It would have more of a rural feel to it. None of the steam punk aspects, more like campaigning in an area that hasn't caught up with the technology. Instead of one central location with the various Houses attached, it would be a series of wildernesses that connected the realms of the elves, humans, dwarfs and so on. You'd have access to some of the crafting places but would have to travel more to get there.
The p2p area would be Eberron. With a subscription you would have access to the warforged race and artificer class. The layout of Stormreach wouldn't change much and it would be more convenient for shopping and crafting - just like it is in a real city. The quest would be more city/sewer based in Stormreach and more outdoor's and cavernous in the rural areas. P2P's would have full access to both but a subscription would be needed to have access to the better technology/crafting.
Races - more or less what we have now. Difference is there would be race restrictions on classes and caps on certain attributes. Humans would have the highest cap on all stats but still have a cap, while the other races would have very restrictive caps on some stats but no cap on other stats and have the benefits of natural abilities and bonuses.
For instance - a human ranger could push all his stats up to 30 (not counting gear bonuses) while an elvin ranger would max out his str at 18, his con at 20 but could push his dex as high as he wanted. He'd trade some stats for unlimited areas of his race. It would give a very different feel for picking race and classes. A elf rouge would play very differently than a human rouge and so on.
Races would lose the option of some classes. A Horc couldn't play a paladin or an artificier. A warforged wouldn't be able to be a cleric, a barbarian, a druid or a ranger. But each playable class would be different by which race you pick. A Horc would start as a basic cleric, but after 5th level it would start a new path specific to Horc's. It wouldn't be a cleric like a human but more of a witch doctor with unique spells for its race/class mix. A Horc would have less healing and more offensive spells than a human and an elvin cleric would probably play more like a FvS.
Classes - characters would start with a basic discipline up until 5th level. You'd start as a basic fighter, thief, magic user and so on. At fifth level you'd be able to pick a more specific path. Fighters would branch out to paladins, cavaliers, or so on. A thief could become a rouge, assassin, acrobat, and so on. Rangers, barbs, and druids would belong to the same basic class before deciding which direction they wanted to go in. Some paths would require more XP to advance to than others (and I'm sure this will be the unpopular part).
In general, the magic user classes would take roughly twice is much XP to advance per level. I'd set it up on the old fantasy settings where a powerful wizard doesn't reach his peak till he's nearly ancient. A powerful wizard shouldn't be easily accessible, he should be a rarity that inspires awe in his mastery of his arts. You should be able to go through 4 or 5 fighter types to have the experience and the strength of a powerful spell caster.
So depending which path you pick would dictate how quickly you could raise through the ranks but also how powerful you are at the end game.
Multi-classes - I'd set it up where you could follow two disciplines at once but it would take 50% more xp to level each side up. You'd be given your XP pool to draw from when leveling and pick which side you wanted to invest it in. You wouldn't have to worry about capping off one side but, like the magic users, multiclassing would be harder to accomplish and earn more respect if you managed to top off both sides.
Quest levels/numbers - At this moment there's far more quest for levels 1-10 than the higher levels. I'd lower the number of lower level quest since you'll go through the lower levels quicker, and put the mass of the dungeons between levels 12-18 since thats where you'll need the XP.
Random Dungeons - all quest would be designed with a random element. There would be a normal setting where, like it is now, the quest could be run with the traps/secret doors/monsters in fixed positions. Hard difficulty would be worth 2X the XP as normal but the locations of the traps and monsters would be random. You wouldn't know where to expect a trap or what type of trap it was so you couldn't be prepared. Since it would be random, there might be a trap down every hallway or none. It would make zerging impossible but provide more XP and better loot.
Elite would both randomly change the locations of the traps/monsters as well as well as how hard they hit. Instead of turning the corner and finding two bugbears like on normal, it might become two (level comparable) beholders. It would be much harder to make it through without a balanced party but you wouldn't be grinding the same thing over and over and elite would give 3X the base XP as well as better loot.
Rares - I'd keep the normal rares but also introduce rare rares and ultra rares. Rare rares would only spawn once every hundred instances of entering a wilderness and ultra rares would be the thing of legends. They'd only spawn once out of a thousand and be the hardest bosses to beat in the game. They wouldn't spawn in the same exact spot but rather in a more broad area. They would be the type of thing you'd take a screen shot of because you know if you claimed to kill one everyone would say 'no way.'