Well, I thought of a character from Xenoblade 1 who could use the monado.... it might be a bit of a spoiler, but judging from the updated Shulk Final Smash, they don't seem to care too much about spoilers
Or they could have Dunban, and his Final Smash calls out Mumkhar and Dickson.
I'd also like to see:
Zach for Cloud
Ken for Ryu
Jeanne for Bayonetta
Shadow for Sonic
*maybe* MGS 2 Raiden for Snake?