That's another reason I was personally suggesting it as a replacement for character DLC. I think all the time they spend making an entirely new unique character could easily be filled with making new costume skins for the characters. They could be simple (giving Link his TP outfit), a "character as a skin" like Alph was (so, Dry Bowser for Bowser), or special event related (Smash Bros Beach Episode)
Also, there's no reason to assume that every single DLC skin pack will cater to every single character ever. Using my joke example (a beach episode), in that scenario, there are a lot of characters it probably wouldn't make too much sense for a whole costume, or if they get anything it'll be minor.
Like, putting swimming trunks on Ridley might be off the table, as hilarious as it'd be. (To reiterate, I'm joking about a beach episode costume pack, but figured it'd make a relevant scenario regardless)
I'm sure if they did something like this (...and considering Bamco is working on it, I'd not be shocked if it happened), the potential sales from the costume packs would be pretty intense.