would it not be fairly cheap to add some simple 3.5 class variants to the base classes? personally, i loved playing a feat rogue. Exactly like a normal rogue but they lost sneak attack and gained feats as a fighter. great for multiclassing. There were quite a few good ideas for altering the simple mechanics of the main classes in several books. I can't say from a programming perspective how time consuming this would be, but it seems that with no need to add any animations or graphics one could do quite a bit for character customization with those rules. The possibilities for character customization was what made 3.5 my favourite rpg ever.
There was a little known variant for fighters that allowed them to trade out their 4th and 8th lvl feats to gain the ability to break things better and damage opponents on a bull rush. I had a ton of fun playing that character, those two simple changes granted my fighter a whole new way to be played and thus rejuvinated my interest in playing a fighter.
Not to get off topic but bull rush and breaking down doors are probably the two mechanics i miss most in DDO.