Well, did Popo and Nana have any ultimate attack moves in the game Ice Climbers? FS are usually extracted from powerful moves or best moves.
In Ice Climber, all you do is jump, shoot icicles, attack with your hammers, and spin around and stuff. However, the final smash should be called FINAL FREEZE! XD
They simply grab the smash ball, and their nB turns into Final Freeze, in which they can use it for a limited time. It is unblockable, and basically unavoidable with the side dodge [since it's so huge]. all you can do is jump over it, jump and air dodge, etc. It's basically the same thing, but bigger, and freezes you a bit longer. That'd be neat!
Or of course, one of the two [Nana or Popo] could go on a crazy suicide strike [and you can't control them], beating the crap out of your opponents [AI controls them btw], at a good skill level, while being super powerful. At the same time, you [the other Ice Climber] are super powerful, and can do the same things and stuff! That'd be intresting=double character power boost!
I think I'll call it Double ****** Strike!