Would this actually be possible, though? I mean, I'm not even sure Brawl4All is that extensive, and I'm pretty sure that that's like the biggest mod out there that adds characters.
Its totally possible now, especially with the DiY codes PyotrLuzhin and I made. tho I do think BeX is limited to 128 characters currently iirc. and ASL will allow up to 2000+ stages max. It is however a lot of work. Lots of smash 4 characters have not had a PSA created yet. Don't let it discourage you tho, the hardest part of building a mod is getting it rolling, and something like this will garner a lot of attention if you stick to it.