Having multiplayer is one of the most interesting features they could add imho. Note that with multiplayer, i mean co-op, play through the main story multiplayer, not some tacked on unrelated multiplayer (like deathmatch arenas, seperate multiplayer maps and such). I have had a really good time playing through the campaigns of Baldur's Gate, Icewind Dale and Neverwinter Nights co-op with my friends.
Not really. Baldur's Gate, for example, is a single hero campaign (with a supporting party), but still, co-op multiplayer works perfectly. Not all players have to have equally important roles