First off, let's identify the problems.
1. in 99% of the game's content, a good caster can take the spot of a good melee, and do better.
2. AC is near-pointless for most builds, and other than getting hit 95% of the time, there is no downside for dropping it completely.
These seem like separate problems, I know. But they're far closer linked than you might think.
Casters should be (and are) more POWERFUL than non-casters. HOWEVER, their power is SUPPOSED to come at a price. They are intended to be FRAGILE. In PnP, the difference between their low hit-die and the next guy's higher hit die makes this difference notable. In DDO, there are more effects overall that grant additional hp, from favor rewards to false life and green steel etc., that benefit both casters and non by the same amount, and therefore, the split between their hp amounts becomes smaller (relatively.)
Now consider the 2nd issue in conjunction with that. The non-caster who is barely less fragile than the caster hp-wise most likely DOES NOT have the AC to mitigate any damage in content challenging for his level (past level 12ish), and thus, can drop as fast as the caster, but has less power to show for his fragility. However, it is important to note here that most of these non-casters can "accidentally" obtain moderate, albeit currently useless, AC values, thanks to things such as building for two weapon fighting, wearing armor, getting passive bonuses from class, prestige, or stance, etc.
It is not the POWER of the caster that is currently placing them ahead of non-casters. It is the fact that they have managed to completely mitigate the COST of being a caster. The largest offender of this is amazing self-healing, such as quickened heals, reconstructs, and negative energy bursts. Whether you stop yourself from taking damage in the first place, or simply deal with the damage after you take it, you have achieved near-perfect damage mitigation. And casters have that territory conquered at the moment. They may simply build towards having "just enough" hp to last between self-heals, and you CANNOT drop them as long as their blue bar has blue left in it.
Now then, we see the problem. Casters are getting what they should get, but without paying what they should pay for it. For the solution, we look at some of the situations where casters DON'T take over, and see why: Big bad hitter raid bosses. Guys that can deal enough damage in 3-5 seconds to overpower the caster's self-healing. There are only a few of these in the game, and they are where you finally want a near 4-digit hp tank that can last between cleric heals, or one that actually mitigates the damage by not taking most of it.
OK so we've gone over what we know, yet again. Time for the fix.
- Add a damage multiplier, applied after DR and other such mitigations, whenever an attacker beats the defender's AC by an obscene amount.
Example numbers: 20% extra for exceeding the target's AC by 20, 40% for beating it by 40, 60% for trouncing it by 60, and so on...
First let's look at this at the intended section of the game, end-game. Let's say we have an epic raid boss with a +90 attack, that hits on average for 100 damage.
Against one of those psycho guys that ACTUALLY MANAGED to get their AC that high? No change.
One of those guys that got decent AC for non-epics? Like the 70's range? +20% damage.
The 50's range is +40%.
Guys all the way down in the 30's range? +60%
And finally those casters that have 10 AC? +80% 180ish per hit. Given a typical boss attack speed, I'm fairly certain I would NOT be able to stand up to that punishment with quickened recon on my Warforged Sorcerer.
Now let's drop down to an example non-epic raid boss, with say, +50 attack and 70 average damage.
epic tanks, high end non-epic tanks, and even guys down at 50, which is achievable without being tank focused, experience no change to their current damage. The guy exactly at 50 would take 20% more damage whenever the boss rolled a nat 20, but thats about it.
30's are +20% yeah yeah
and the 10 ac casters take +40%, so about 98 average. While this number per hit is certainly not deadly to many end-game casters, it does start to approach the territory of "quite irritating," forcing them to hit their self-heal more often, and thus use up SP faster for doing something they really shouldn't be: being the guy getting hit.
While the raid boss situation isn't all that impressive, rethink those same bonuses on say, a troupe of devils in Shavarath with their wonderful attack bonus, against a caster doing the typical caster thing and soloing the quest.
Now for the lower levels.
Low ac builds would experience a good deal of extra damage in the vale and gianthold, but as you go lower in the game, the amount of enemies who can defeat your AC by an amount necessary to net bonus damage becomes very slim. To the point that even a 10 AC character in the first few levels probably won't experience it.
While the numbers above are guesstimates, I believe the general premise to be promising. The goal is to push casters back out of the tank territory by making it less efficient (and at the high-end, less possible,) without flat-out forbidding it (casters could make builds with some concessions for AC), and without damaging melee as much. Seeing as how most melee, thanks to their armor or wisdom, class features, and the like, can hit moderate AC without any real concessions away from their build, a system punishing super-low AC seems the best way to do it.
There are also 2 side-notes.
1. Applying this system to the players would be another advantage to non-casters. Players could then deal bonus damage for defeating an enemy's AC by a significant amount. This could make to-hit bonuses more tantalizing, as the only consideration now is "do I hit on a 2?" And seeing as how casters don't make attack rolls, they don't really benefit from this. They would however make **** sure all the melee are GH'd and hasted for +5 to-hit, possibly pushing some melee over the edge of the damage bonus. Symbiotic relationships between classes are good!
2. This suggestion would most likely put quite a few people in the position where they want to LR/GR to fix their character to account for it. While this is something I WOULD like to avoid normally, I realized something earlier today. We're about to have all our enhancements reset and changed. People are about to be thrown into that situation whether this suggestion goes through or not. That would be an excellent time to fix a long-broken mechanic that is causing such imbalance between two main character types.
Welp, there you go. I'm sure I'll be kicking myself when I can't use my warforged sorc to solo everything ever later, but I believe that I shouldn't be able to. At least not so easily.