The easiest way to get away from a standard Fighter/Mage/Thief paradigm where each role is played by a dedicated player, is to have a party with classes that can perform multiple roles teaming up. So you might end up with two classes that can be the "thief" role (like a Bard and a Ranger) teaming up, but who can also do other things like tank on the frontline or work a series of wands to be the mage. In a party like that, you might end up with some overlap in roles, or you might end up with roles that aren't covered all the way unless two or more members work in concert. Either way, you can get some variety.