For a while I've felt the inherent problem with adding new human characters is that they'd feel too "fanficy" which is why they're seldomly added if at all.
That and EPD's design philosophy emphasizing gameplay before everything else.
Daisy only exists as an extension of Super Mario Land taking place in a foreign locale and was later bumped up for the roster in Camelot's games where she just stuck.
Wario likewise exists because they wanted to do something different and not be restricted to past Mario titles and this extended to presenting Mario with an objective about reclaiming something that was his' rather than fight on behalf of someone else.
Both these characters were made by R&D1
New Donkers in Odyssey are deliberately meant to be odd looking to sell shock value to the player.
But I have to ask, what was the reception to Rosalina upon first being revealed?
I know she's a massive fan favorite these days and I see her appeal but how did people feel about her upon introduction pre-release?