HEre is the list of possible Final Smashes for him...
1. Maximum Tornado- This move seemed like the top priority for him until we got his moves, which one of the m involves the tornado move. This doesn't rule it out though. This would be much larger, and possibly not surround him.
2. The Metaknights- Metaknight is the leader of the... Metaknights. He could indeed call upon help from his sword wielding friends. This move would defiantly be like Pit's. I'm afraid Final Smashes will inevitably be somewhat same. Like Mario's and Samus's are blasts, and Pikachus, Yoshis, and Diddy's involve flying around. And i can assume Bowser will not be the only one with a transformations.
3. Sword Beam- This was one of Metaknight's signature moves. It was excepted to be one of his moves. It was pretty big in the show, so it does seem that Sakurai held it from his moves to have it at full effect. Whether its one beam or multiple, this could work, easily.
I can't really imagine it being anything else from the above three.
Lol, all these ideas are probably have been said in this thread before, now that i think about it.