As far as monsters go, I think starting the game off with lesser monsters like goblins and such is a great idea, the devilkin in d2 did a good job of that. I think it would be much better if natural creatures such as wolves and rabid animals is a cool idea also. I just didn't like the jump in act 1 from quill rats, to goblins and zombies, to courpted elite rouge warriors.
I think D3 has a lot of things from D2 to improve upon, and monster variation is one of those things.
As far as bosses go... I haven't read any Diablo novels or fan fiction, but I will give it a shot. Since Diablo, Mephisto, and Baal are all "destroyed", and they were all brothers, why not have another family member interject? Mommy and Daddy could be pretty pissed to find out all their sons have been destroyed, so why not playcate of that idea?
Well... thats all I got... :dance <---
