They should be doing DLC MK8 style.
Cool stuff not relevant to Nintendo (Snake, Rayman, Shadow, etc) - Like Link and the Villager in MK8
Characters not really justifying their own position (Daisy, Cranky, Ghirahim, Adam Malkovich, etc)
Clones - (Dr. Mario, Lucina, Dark Pit, Ninten, etc) - Dry Bowser, Cat Peach, Tanooki Mario
That's not to hate on the above. I would have bought Lucina (2noob4Marth, and those alt colours). Heck, price dependent I may have bought the lot.
But DLC should not be used to get highly demanded veterans back in. It should not be used to give franchises proper representation (Donkey Kong), or any representation at all (Golden Sun). Now I would buy Mewtwo. On Wii U. Not on 3DS (unless crossbuy is enabled). Not on the Nintendo Fusion (or whatever) for Smash 5/6. Nintendo get more of my money in the short term, but lose out in the long term.