My ideal order would be like so, personally:
Mario Main Series Protagonists (Mario, Luigi, Peach, Rosalina, probably Captain Toad if he's in.)
Mario Main Series antagonists (Bowser, Bowser Jr.)
Mario Spin-Off Characters (Dr. Mario)
Side Series Protagonists (Yoshi, Wario, maybe Captain Toad if he's not considered a protagonist, per se)
Donkey Kong Main Series Protagonists (Donkey Kong, Diddy Kong, Dixie if she's in.)
I put Donkey Kong separately because I consider the Mario and Donkey Kong franchises to be two distinct main franchises, while I consider Yoshi and Wario to be Mario side series.
What if this is the first game where the non-surprise retro character isn't from the NES era?
That's certainly possible, but Takamaru does have an impressive resume.
Maybe Muddy Mole as a Gameboy retro?
Why Takamaru? Why choose a Japan Only character when there's Balloon Fight and Excitebike? Excitebike was a launch title and one of the ten most successful NES games of all time.
I'm glad you asked, actually.
- The Mysterious Murasame Castle is the sister franchise to Zelda, having used the same engine similarly to how Kid Icarus used Metroid's engine.
- The game is considered a cult classic and a must-have game for the Famicom in Japan.
- The Mysterious Murasame Castle has been released overseas via the Virtual Console since Smash 4's roster was decided.
- The Mysterious Murasame Castle receives an attraction in Nintendoland.
- Sakurai revealed in interviews that he had considered Takamaru for Melee and Smash 4. He even said that people from his generation of gaming request him regularly.
- Takamaru appears as an assist trophy in Smash 4.
- Takamaru received a Mii Costume in Smash 4's DLC. The sales would help to gauge the West's interest in the character.
- Takamaru seems to be more popularly requested among the Smash community. If this is true among people who participated in the Smash Ballot, Takamaru may be considered due to his relative popularity among retro characters.
- Platinum Games' Hideki Kamiya, a peer of Sakurai and the director for Okami, Bayonetta, and NieR has expressed multiple times that he would like to work on a Mysterious Murasame Castle revival game. Sakurai including the character could be done as a favor to Kamiya, raising awareness of the series outside of Japan. It would be characteristic of Sakurai, as he is on record saying that he enjoys seeing older game franchises revived revisited for modern audiences, ala Kid Icarus.
That said, Balloon Fight and Excitebike both have notable resumes as well. I just consider Takamaru more likely for this particular game.
Also, the game's about a Samurai/Ninja who saves Japan from aliens that resemble creatures from Japanese folklore. The implication in the game's canon is that Yokai in folklore were actually aliens that visited Japan in the past. It's kinda amazing.