They made Agitha work, so I'm down. Not gonna lie, kung-fu shopkeep would've been fine too.
I doubt it's possible at this point, but it would've been wonderful if the animal trio from the Oracle games made it in. A boxer in a Musou game of all things would've been hilarious if nothing else. Of course, Din and Nyaru would most likely be considered first.
I'm still holding out hope for a full sequel for the NX. That would be a good opportunity to add in the Oracle games and Minish Cap. I would love to see Ricky, Moosh, and Dimitri as characters or weapons in Hyrule Warriors.