I'm not really sure if I can give good arguments for ALL of them, since a lot of my most wanted characters are kinda just smaller first party franchises I have nostalgia for and third parties I liked the games of (Digimon) or think would be funny (Monokuma). Like I consider myself pretty lucky in Ultimate because I was kind of on the Banjo train but didn't know how likely it was because he was Microsoft owned, so when that Jiggy bounced across DK's house I actually screamed out loud IRL lmao
And then I ALSO got Pyra/Mythra later on (I remember I had to work that day and I looked at the website while I was at work and saw Pyra and Mythra and was unable to focus the rest of the work day lol), so I kinda made out like a bandit in Ultimate DLC.
With that said, I think my biggest want right now is a Rhythm Heaven character. With the plan for next Smash likely coming soon if it hasn't already happened, I genuinely do feel like the announcement of Rhythm Heaven Groove is happening at the perfect time to potentially put that idea that got scrapped in Smash 4 back in Sakurai's mind and makes it an ongoing series again instead of having to hope he goes back to a franchise that hasn't had a game in a decade.
Second is definitely Bandana Dee, but I feel like he's one of the more... standard picks out of characters I like? As in like, most people think he has a pretty good shot this time around I think? He's the last of the Return to Dreamland crew, his role has been on the up and up lately to the point where in Rainbow Curse, Battle Royale, Fighters 2 (at the start anyway), and Forgotten Land he's Kirby's Player 2, I could see him potentially be playable in Air Riders, though obviously that depends. We still don't really have many spear fighters since Byleth only uses a spear like 1/3 of the time, I'd love to dive into Dee's history and pull out Megaton Punch as a Final Smash, and I'm a huge Kirby fan (duh) and was always gonna root for Bandana Dee lol
After that is probably Noah or Mio. I've said before I don't think both is happening, nor do I think we'll get a class change system since I feel like that would just be kind of odd. If I had to pick one, I'd prefer Mio I think for her unique weaponry, but Noah is probably more likely as the holder of the plot sword and could potentially have a mode or "install"/Limit type thing for Lucky Seven's alternate attacks maybe? So I still think he could be an interesting character in his own right. I don't personally see a brand new Xenoblade happening in the interim, but otherwise I can't really explain this one too effectively minus I just love the **** out of Xenoblade 3 lmao