Are you talking about games like Diablo and Baldur's Gate? If so then no, there's no online games like that on the wii. You can count the wii's online games on one hand... one deformed three fingered hand.
However, there are some good hack and slash/dungeon crawlers on the wii. Marvel Ultimate Alliance and Alien Syndrome. Both are multiplayer, though you'll have to share ass-space with your friends, but they're both loads of fun.