I wouldnt mind seeing a limit on items to 1 per class (1 boot, 1 helmet, 1 chest, 1 glove, etc...) This way a DG would still be able to customize itself, but not really stack 3 +HP/Armor chestpieces. Its not really a problem, because any DG can do it. I know my Rook usually has 8k+ mid game (level 15 or so) because of items. I tend to not use towers much, so I dont need mana much either.
Not every DG gets the same advantage in HP stacking. The thing is that the UB can get very good speed and damage without too much effort in terms of items. His only weakness is low HPs, so the standard thought is that yeah, hes fast and will hurt you, but you can kill him yourself very fast. But he can patch that weakness with items late game, then you have a DG which is pretty fast, does a lot of damage, and is really hard to kill. Many other DGs just cant reach that level of deadliness even late game.
I think that the idea of having a HP ceiling is a good one. Just make it some constant factor of the original hitpoints each DG has. That would allow something like say, a Rook, have a much higher HP cap than a UB.
Really its becoming pretty apparent that HP stacking is a bit overpowered. I think that a very slight HP reduction on a lot of items might be in order too. I personally find it a bit boring in games where everybody has HP stacked right away. Nobody dies and the game just drags on. I suppose some people will disagree, but thats how I see it.