Hard to think of five given they covered so many... I'll go outside of Nintendo with a couple of these

1. Captain Toad (no idea how this would be possible heh)
2. Rayman
3. Banjo/Kazooie combo (I recognize this is impossible)
4. The knight (Sir Arthur) from the Ghosts and Goblins series
5. Simon Belmont (Castlevania)

I would say Toad/Captain Toad but I feel that would ruin the happy feels you get when playing other games with Toad aka Captain Toad/ Mario Kart.

???? If so why hasnt MS said anything to Nintendo yet??? :0

Edit: In case you were wondering about my comment, King K Rool already appeared as a trophy in every SSB game, he was playable in a wii spin off. He's owned by Nintendo.

Simon belmont is possible. I see it happening in my crystal ball :---) castlevania needs a push. They just have to skip giving him the fetish wear, as it would possibly impede on nintendos "image".

Rayman is a trophy in the game so that might be possible. It's unlikely many people would even know who Sir Arthur was, though, and I'm not sure how they'd incorporate his armor leaping off bit into the game heh

Unfortunately there's no way in heck Microsoft would let them use Banjo/Kazooie or Battletoads... Just a little wishful thinking.

I thought that is the reason we didn't see them in Returns, but I could be wrong and lazy to do some research

No Top 5, but probably one or two Chars of Splatoon.

Retro created an entirely new set of enemies in dkcr and dkrtp. To have a kremling or king k roll suddenly appear would conflict with the themes of the games.

King K Rool is my most wanted

Other than that I'm fine with anyone, but I don't want someone in particular.

sure, but they did that because Kremilings belongs to Rare (Microsoft) right? So is impossible to see King K. Roll