I don't live in NA so I don't have any interest in the competition but I'll have a go cause why not.
1. Joker + Mementos, obviously.
2. DQ character + Whatever DQ stage is appropriate. Not huge into DQ so I don't understand specifics but what I've followed of SSB along with how DQ11 & DQB2 are releasing on Switch soon make me think this is kinda inevitable. Plus it's a big franchise relatively speaking that isn't represented yet. Very confident in this.
3. Doomguy + Hell. All bets are off I feel at this point, Smash is so ubiquitous that most of anything can be at least feasible, and with DOOM + Eternal being on the platform, the series being iconic and it being an unexpected/controversial pick I feel like it would be a good pick for the 3rd one, generating guaranteed headlines halfway in.
4. Pokemon from Sword/Shield + whatever gym/Pokemon League/route area in Galar is cool looking. I don't think it'll be all 3rd party characters and this seems like the sensible choice for a Nintendo character. Wouldn't be massively popular considering how many 'Mons there are already but it's a safe bet with synergy with the Pokemon games themselves releasing late this year.
5. Professor Layton + St. Mystere. No real logic or reasoning for this one, just really, really want it to happen. I've been on this train for a decade now and I ain't getting off until the right thing happens.