For Medusa, I've grown to want Medusa a lot more. She's the missing original trio of KI characters, where the other 2 are Pit and Palutena. She's a villain, a female villain at that, could be the first one ever in Smash (Dark Samus isn't a "she", it's more of an entity or manifestation of something, so it don't count). I'm certain she can be unique enough to not be an echo of Palutena if we look at her various moves from her KIU boss battle, featuring attacks from her eyes, poison from her hand, maybe something with her snake hair (though I would want costumes of her having black hair as a reference to her Medusa's Revenge incarnation, prior to being turned into a monster), nail claws, and more, which could make her very different from Palutena herself. I wouldn't be disappointed if she was an echo in a way though as long as she has many unique moves.
The 3rd party member has to be Monster Hunter for me. I mean we got the Rathalos as both a boss and Assist Trophy, where it was shown on the A. Steppe map from MonHun, too. At the same time, there were Rathian/Rathalos Mii costumes back in Smash 4, so surely they could do something to bring in Monster Hunter, well, pretty much the Hunter, to us. MvCi did it, but I'm sure Smash can do it better, including giving us 4 male and female outfits and different voices to choose from as a reference to customizing your own hunter. I really would want to see this work. Bonus points if I can use the Kirin Armor. <3
For Phosphora, let's just say I have KI bias. We've seen her fight, and she has plenty of potential based on what we've seen and know alone from her Boss Battle, or well, the whole chapter dedicated to her. Fast, aggressive, but could get weaker overtime from releasing so much electricity unless she recharges (which will be a move). She'd also be a suitable choice to bring one of the Forces of Nature members (I don't count Dark Pit) to be playable as Viridi is already in the background of the Reset Bomb Forest stage, and the most capable of Nature members.
To add to this, the fact that she suddenly disappeared as an Assist Trophy in Ultimate, I think, as well as the disappearance of many other Kid Icarus things, does seem suspicious. Like with Little Mac and Dark Samus, I'm hoping that Phosphora will be another Assist Trophy to be promoted into a playable character.
Unfortunately I don't really generally care or have interest for many other characters at all, and can't list others besides what I listed. These are the 3 that I actively support, but do try to give fair reasoning and such to back up my support.