Well, if they did implement gunpowder and rifles in Diablo 3, I had had a few thoughts about a Ranger-style character, who's emphasis was on stealth and his primary weapon was a rifle, think like a sniper.
He'd have varieties of coercive spells which could be used to subvert his enemies temporarily or permanently, have passive attack bonus upgrades for using long-ranged weapons (bows as well as rifles). They can use certain melee weapons dual-wield, but only certain types of weapons.
The very gameplay itself would be different, as the environment on-screen would have seperate advantages on the map that would "pop up" for Rangers only (reflecting their training in Westmarch). Examples would be places to ambush demons and such, or places that (via vegetation or something) would be like the D3 red-floating-bally system of healing. One could transfer the D2 Barbarian's "Find Potions" and "Find Items" skills easily too, as they are very, very observant.