I feel like the series' roster is extremely sporadic and has some… Questionable choices. I don't understand why Bowser Jr. and Waluigi weren't in 7, yet Honey Queen was. I hope that for the next entry, they focus more on fleshing out the roster, and going from there. I feel like Wii's roster, plus Petey, Lakitu, and Shy Guy would be a great start. I'm not opposed to non Mario characters being in, as long as they are a small portion of the roster, I feel like adding a lot would be like adding a lot of 3rd parties to Smash, limiting the Mario/Nintendo characters that can be in. I'm also not opposed to the Koopalings, but they take up too much space, since you can't just have one. I feel like they should be themed DLC.
One thing that would also be nice is if the roster reflected that generation's Mario game. 8 should've had Nabbit, Boom Boom, Pom Pom?, Captain Toad, Wingo?, and the Sprixie. 9 should have the rabbits in Mario odyssey, or if there is a leader, him, and so on. Being said, I'm not disappointed with 8 Deluxe's roster, since I feel the returning characters were fan favorites, and Inklings advertise Splatoon. These are just my thoughts om how rosters should work.