Putting Geno in would be purely fanservice; he has a role in one video game, as a party member, with a cameo in another. Krystal, I agree, would be a good addition, but to be honest I wouldn't have her replace Wolf, but Falco. Falco's an incredible character and all but he has even less moveset potential than Wolf. I could see him as a top-tier Assist Trophy.
But I agree with Glorb about Ridley; he's fanservice too, but has much more reason behind it. He's appeared in many more games than Geno, and would cater to a lacking Metroid representation (especially since Metroid had begun to rejoin the mainstream with the Prime series).
(I heard a rumour that Ridley is unlockable if you beat SSE with Samus, but whether you can actually choose who to beat it with is an important but still unknown factor)