You know, I don't think being a Spirit makes it impossible for a character to be DLC. They can always just remove the spirit after adding the new character. If it is a World of Light spirit, they can just replace the spot where you got the spirit with a spot where you can fight the actual character. And since there are hundreds upon hundreds of Spirits, losing one doesn't make a major difference to the game overall.
So I don't think being a spirit deconfirms a character's DLC potential, but I do think that people should temper their expectations.
I think if we have a franchise represented by spirits, but not a fighter, and there is a major character from that franchise not represented as a spirit in the game, such characters are the most likely DLC candidates. For example, we've seen several rabbids as spirits - has anyone seen Rayman?
If Rayman is not in the game as a spirit, I think he'd be very likely as DLC.