Sheeeesh.
One of my closest friend, he got 2 characters as his most want;
1. Krystal from Star Fox. He had Adventure on his GC and have been asking for her ever since. I know more people asking for her, and that her best shot was back at Brawl, but for them I'd like to see her, but personally, I prefer Wolf.
2. Banjo & Kazooie, another game he had back on the N64, and while they have been in Microsoft hell, he still held out optimistic about their inclusion, and for that determination, I'd like to see them added for their fans, I know it's been a struggle.
Ridley - While I've opened more for Ridley in later times, I still don't hold a specific attatchment for him. I respect his role as a Metroid villain, and one of Nintendo's most notorious villains in general, and it would be nice to see him for more Metroid, and for the fans of his which have suffered all these years. I think I would like to see him myself, but he is not one of those I die to see, personally.
Paper Mario. I really did not like the idea of yet another Mario as Doc returned for Smash 4, but I know PM is different, and that he would bring something unique, but I just can't be much bothered myself, but for the fans of Paper Mario, I'd like to see him added, I know they have been waiting patiently, and who knows, I might be surprised.
Anything from F-Zero. The fans have been left in the dust for too many years. Anything at this point would be great. Black Shadow, Samurai Goroh, even Blood Falcon as a clone character ...
I'm sure there are more, but those come to mind.
There are some I'll quickly mention, some that I'm also wanting personally, that I know the fans would be really glad to see!
King K. Rool & Dixie Kong. Long time fans of Donkey Kong knows the struggle.
Isaac from Golden Sun, would please the fans of Golden Sun more than one could imagine.
Lip, people who have played one of the Panel De Pon type of games have hold out for any recognition for their Lippy.
Geno. Perhaps a small vocal base, but a faithful one which have kept faith with them, it would be nice to see them smile.