If we get DLC fighters who are a base game spirit, what would happen to their spirit battles?
If their already existing spirit battle is kept even though they are a spirit, it wouldn't make sense because they would have another fighter has a puppet fighter while the spirit is an actual playable character. To add on to this, if the DLC character has a fighter spirit, (Which is very likely considering there is no Piranha Plant spirit yet, so hey could be waiting to add him as a fighter spirit when you get his DLC.) there would be duplicate spirits, one fighter and one normal. So it could be possible to use the DLC character with a spirit of themselves.
Now what if they removed their spirit battle? If they did this, then they would have to remove the spirit as well. They could, after you purchase the DLC, take away the select spirit battle and spirit and give the player some SP, Snacks, another Spirit, or Spirit Board items to make up for it. IMO this is probably the only way we could get spirits as DLC fighters..
And what if you can't use DLC characters in Spirits mode? I don't see them doing this, but it is a possibility. There would still be a duplicate spirit problem unless the DLC character has no fight spirit. (Which means no special classic mode reward.)
I think there are ways to get around this pretty easily for most characters with spirits.
Note, for example, that Link has multiple spirits which are just versions of Link from different games. His fighter spirit has the BotW design and his Smash render.
So, suppose they added Dixie Kong as a DLC fighter? Well, they could just rename her spirit to reflect which game the artwork came from, while her Fighter Spirit could either:
1. just consist of her Smash artwork (like Cloud, the Wii Fit Trainers or the Mii Fighters)
2. use different artwork for her Fighter Spirit (if her current Spirit is from DKC2 or 3, then use Tropical Freeze artwork, or vice versa)
The first solution would work for any DLC character, while the second would work for any with many game appearances (and thus many options for the original appearance Fighter Spirit art)
It is pretty easy to accommodate a character already having a spirit, although I'd certainly consider it to hurt their chances significantly given that they have already chosen the DLC before launch. The devs would have had to make a conscious choice to somewhat mislead us - they would've known that the "Dixie Kong" spirit would need a different name after Dixie Kong was added as a DLC fighter. But they would've also chosen not to name the spirit "Dixie Kong (Donkey Kong Country 3)" or something because they would've known that would lead a lot of people to conclude that she was going to be DLC.
So, they can deal with it... but would they have chosen to do it that way? Maybe not.
(Also Piranha Plant is not a good precedent since they announced it before the launch of the game and reveal of the full spirit list. There was no reason to play coy as a result. And the absence of a Piranha Plant spirit wouldn't have been that suspicious anyway, although it would've been seen as strange people probably wouldn't have seen it as indicated it would be a fighter...)