DLC Job 1: Waddle Dee (Kirby)
I attempted to get Waddle Dee in once before, as a late stage derivative a la Jigglypuff. That didn't pan out, but now might be an even better time. Piranha Plant has kind of set the standard for bonus fighters in my eyes, someone a bit less conventional or exciting in and of themselves who still adds something flavorful and surprising to the roster dynamic. Hence, I will continue to follow its example and nominate a generic enemy. If it works it works.
Although the thing about Waddle Dee is this, they are probably the most heavily merchandised element of Kirby outside of the pink blob himself. They have their own portal on the official Kirby site alongside Dedede and Meta Knight - not Bandana Dee, just "Waddle Dee" as a whole. Of course Bandana Dee would be represented here as well, but probably as a costume alongside Sailor Dee and so forth. I also assume we may already have Waddle Dee models in the game, as part of Dedede's entrance and victory animations. So this would be an easy asset flip, represent an important part of a popular series, and frankly I think may just be a fun fighter in their own right whichever direction we want to go with it.