Actually, I made this instead.
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By pure fan demand, Geno is easily #1.
2B could be the equivalent of Joker in FP1.
Crono would be a retro rep.
Neku would be a promotional rep for TWEWY 2.
Agnes would also be a promotional rep for Bravely Defualt 2.
And Lara would just get in because its freaking Lara Croft.
Geno has the issue of being really "unimportant". The other MW characters like Chrom, Daisy, Banjo, K. Rool, Ridley, Isabelle and so on are all important or recurring aspects of their respective franchises. And is last legitimate appearance was in the 90s. The fact that Geno has a mii costume in Smash Wii U is impressive on its own.
2B has the issue with her outfit. Unless Sakurai changes his mind, I can't see him censoring 2B as her creator would probably find her censoring to be disrespectful to the character. If Mai didn't even get a spirit (even if she's essentially the Luigi of SNK as a whole), 2B's costume could make things complicated.
Crono might be too visually and mechanically similar too Hero to the dev team. And more importantly, Square just doesn't seem to care for that IP (you could say the same for Banjo but Crono has never had profound fan demand).
I really don't see any barriers for Neku besides just being a small IP.
Same goes for Agnes.
Lara Croft has the issue of her franchise not being huge in Japan. Now, its not that Tomb Raider doesn't do decent in Japan (Rise of the Tomb Raider sold like 21,000 copies in its first couple of weeks in Japan) but its not huge in Japan. Though, maybe Lara Croft could be a reverse-Hero situation. Where Nintendo and Square attempt to put more attention on the Tomb Raider IP in Japan. Idk.
Yeee