1st Party
1.- Bandana Dee; because he's popular and has a very nice niche with him
2.- King K. Rool because he deserves to appear alongside with other Nintendo Icons, and he has a very potential for his moveset.
3.- Elma, I think she would gives us something very special for the roster with the overall way she is designed (I would like to see more moveset, but I would like to see her in the game).
4.- Captain Toad, he essencially represent another Nintendo Icon, and has a very cool game to represent (also, put him a Toadette Alt please, I would play as her always).
5.- Dixie Kong, more Nintendo Icon, that 4th DK character the franchise deserves, I'm not that fond of her, but she kind of deserves it.
6.- Lip, I think she should be our legacy character this time
7.- Isaac, Golden Sun it's a franchise that under other circunstances would have get a chance in Smash, such a shame we was overshadow.
8.- Springman, obligatory new IP character, also, having long arms is fun.
9.- Rex & Pyra, such a breakout duo that it would gives us something very interesting in the franchise, at least gameplay-wise.
10.- Micaiah, my biased choice, but the one will try to defend everytime someone asks me why I want her in the game beyond just my attachment.
3rd Party
1.- Simon Belmont, the representative of Castlevania/Akumajo Dracula, one of the most iconic game franchises.
2.- Rayman, kind of iconicic and related with Nintendo Story thanks to Ubisoft's colaborations
3.- Crash Bandicoot, such an icon, known character
I don't have idea which to pick for the next spots; but I'll mention Sora, KOS-MOS, Geno and Heihaichi as honorable mentions.