From what i've noticed and experience through these games is that diablo sorta has this subtle evolution of depth. D1 was based on a 16 level labyrinth in the tristram church. in 2 the quests were divided into acts. plus they added new classes and various new weapon options and even skill class options. items and everything was randomly shuffled. For D3 I would take this a another step further. basing D3 several decades after d2 as a world divided by 3 regions. each region would be a cluster of 4 acts. each act would then have 9 quests. giving you a total of 108 different quests. this way the game isnt rediculously long but still very vast. also, it would be cool to have optional quests and required quests to fullfilling that particular act. For instance, if you wanted to help out a villager as a optional quest you could do so an earn a unique item from him, where as a required quest would be like pulling a switch or fighting a boss that unlocks the next quest or entry to the next act. also, one of the things that kinda bugged me was the way point teleporting. etc. I mean dont get me wrong, it was always very useful having a town portal on hand if your inventory was full or when you would get bombarded by eniemies, its just I hated how the way points seemed automatic rather than earned. also the fact you can go from start to end in a few clicks. it would be interesting to find the way points locked and you either have to find a key, stone or fight a unique monster or boss character to unlock entry into the way point instead of just having your character walk over it and your done!