I've always wanted to try a mad scientist type experiment: Take a fixed gear hub (which can accept a standard freewheel) and put a multi speed freewheel on one side. Then get a longer axle and space the heck out of it. You'd end up with a multi speed wheel with NO dish. Of course, the dropouts would have to be mega-wide, but with some tandems already at 160mm it shouldn't be too bad
Of course, I'll NEVER get around to doing this, but it *sure would be fun*!