I think the “who” of the roster should come after having a strong core ethos, theme, gameplay choices to the game, aside from obvious ones like Mario, Link, Pikachu, Kirby, DK, Samus, etc. Ultimate Deluxe can just tack on extra characters I guess, soft reboot with a focus on reworking the cast can consider both uniqueness and big names with a lot of potential, a “celebration of gaming history” or “Nintendo history” focused one can consider those factors + different points in gaming history, genres, even consoles and types of ways to play games (handheld, mobile, arcade, PC, etc.), all that. You can probably tell what options I consider more interesting.