Oh, absolutely. The things people did in the Brawl days was absolutely amazing. My only concern would be that there aren't enough people who have an interest in modding SM4SH Wii U, especially now that the Wii U is at its "end" as far as Nintendo, and indeed the general public are concerned. But then again, the game is far easier than Brawl to mod, and people are more experienced this time, so I guess it evens out. Best case scenario, the NX is a success, SM4SH is rereleased on the console with the same file formats as Wii U, and an exploit is found on a current firmware which Nintendo doesn't patch for a few months. The would be a Godsend for the modding scene IMO.
However, another problem is animations. Right now I could add a second slot on the CSS for Fox, rename it Wolf, change his default colour, the colour of his Landmaster, change his weight and speed, as well as the damage and knockback his attacks would deal, but unless we have custom models and animations, new characters are a no go.
EDIT: Aayyy, we have the same post count.