It should be mentioned that there should be no non-video game characters. Sakurai already mentioned this at the recent Famitsu awards show. He's not gonna add the likes of Goku, Shrek, Luffy, Naruto, etc.
My six picks are:
Rex/Pyra/Mythra (Xenoblade Chronicles 2)
Chun-Li (Street Fighter II)
Urshifu (Pokemon Sword and Shield)
Travis Touchdown (No More Heroes)
2B (Nier Automata) (they would really have to modify her skirt ala Palutena)
KOS-MOS (Xenosaga)
I don't expect either of my picks to come true, just wishful thinking. It really doesn't matter to me in the end as I trust Sakurai to create 6 unique fighters.