there is one way to get some of 20XX onto project m (specifically stages, a few mechanics, and characters)
1. Rainbow FD with Alts (DL for Rainbow FD:
http://forums.kc-mm.com/Gallery/BrawlView.php?Number=207651)
2. Smash 3 Fox/Falco (DLs:
http://forums.kc-mm.com/Gallery/BrawlView.php?Number=35940 and
http://forums.kc-mm.com/Gallery/BrawlView.php?Number=202897)
3. Mushroom Adventure (DL:
http://forums.kc-mm.com/Gallery/BrawlView.php?Number=209209)
(it goes over FD but i believe you can adjust the coding to allow it to appear as an alt for FD or replacing online training mode or something)
4. Alt Characters (if you have project m 3.5 or 3.6, that problem already has a solution):
Press L in CSS for Giga Bowser, SoPo, Sheik (already her own slot), Wario can transform into Warioman, and if you decide to use Project M EX, than you can make Master Hand, Crazy Hand, and possibly Tabuu appear lol.
5. Stage Alts (Project M 3.6/Project M EX allowed you to use alt stages. Plus there's the classic stage expansion pack, and i believe there was a build that incorporates the stage expansion pack/project m 3.6's Hold L, R, Z, etc to load a stage
in all seriousness though I know that Battlefield had alts, FoD had a lagless/omega version, FD had a Sm4sh, Rainbow, and other alts in 4.05, and i believe there's a Tourney Legal Hyrule Temple (balanced hyrule temple possibly) available on the vault. so at least a few pretty big characteristics of 20XX can be imported into project m at this point.