I actually quite like the choice! I dont feel a thing for the character but Ive always thought the spinoff games should have this sort of configuration where they have the main ones stay and then alternate the rest of the characters depending on the generation. You know, how GameCube era had Petey Piranha and King Boo, then Wii gave us Rosalina and Queen Bee? I think thatd keep the roster fresh every time and give an excuse to make a new game. Make it different, not try to make it the ultimate one.
I agree for the most part, thought I think there is a way to keep rosters fresh, while also coming closer to what one would consider the "ultimate" roster.
As an example I'l bring up Mario Golf World Tour, because I think that it has the best roster we had in forever.
You have plenty of returning characters in the form of:
Mario
Luigi
Peach
Daisy
Yoshi
Birdo
Wario
Waluigi
Donkey Kong
Diddy Kong
Boo
Bowser Jr
Bowser
Only Petey, Shadow Mario and Koopa (as well as Neil and Ella if you count the Advance Tour characters) were cut, the newcomers are:
Toad
Toadette
Paratroopa
Rosalina
Kamek
Nabbid
Gold Mario
Each of these characters has something that makes them appealing as Newcomer, Toad despite being a main character, hasn't been playable in a Camelot made sportsgame since 64 Tennis, so that speaks for itself. This is the first appearence of Toadette after 5 years of absence, so she really hasn't been in rosters for a while, Rosalina and Nabbid have never been playable in sports games before, Kamek has only been playable ONCE in a spin off before, despite having been around since 1994, Paratroopa is a bit of a refreshing mixup from the typical Koopa we usually see in spin offs and Gold Mario actually plays differently from the cast, in a way that represents his powerup, making him a really unique character, despite just being just another Mario.
Mind you I do think that Mario Kart could benefit from that kind of alternation, especially with the atrocious clone baggage that it brought onto itself, I welcome a lot of the new characters like Luma in Mario Tennis Open, Nabbid in Mario Golf World Tour and now the Sprixie in Ultra Smash with open arms and would be totally down with those kind of charas taking up larger parts of the future rosters, but I feel that there are plenty of instances where some of the more usual characters are just as good, like Donkey Kong returning to playable status in Mario Party 10.
You might not agree with opinion my opinion on World Tour's roster and that's fine, but I just feel that it knew how to make a compelling roster, (even thought the only new added character from our current generation was Nabbid) especially thanks to it's timing, Rosalina wasn't quite as much of a guranteed inclusion yet, we knew nothing about any other future Toadette appearences (other then Mario Kart 8 ),etc,etc, but it's because of said timing that they still felt fresh, despite being older faces and you can do the same thing with characters like Petey or King Boo, bringing them back to playable status after almost 8 years and having them co-exist with new faces.
...I really hope I'm making sense, my main point is that you can have relativly few new faces, while still keeping the character selection fresh, without abandoning too many charas from previous generations.
With that said your idea is perfectly sound and I really hope I didn't word myself in a way that makes it seem like I'm talking down on it, if I did feel free to call me out on being an asshole.