MnSG
Cosmic Beauty
That's a very solid roster and something I'd probably bank on. It's not stuffed to the brim with too many generic enemies and it has a nice mix of newcomers and returning characters.
Honey Queen seriously got shafted in terms of returning characters and her negative reception for in Mario Kart 7 isn't even her fault, it's because of the cut characters. She's otherwise a very solid, left field choice, a memorable character, from a very popular game that fills the role of a large, super heavyweight female character who is also a nice and gentle character as opposed to the rowdier super heavyweights.
I'd also support a Sprixie Princess being playable, and they have their alts being selectable costumes, though honestly, 3D World is kinda old as opposed to something like Odyssey so I'd want someone like Cappy being there first.
I'd also like Wiggler to return too, very unique and fun character to have around. The turning angry aesthetic gimmick is a neat thing to have.
To be honest, as much as I would like to see King K. Rool, I've had to lower my expectations, knowing that he hasn't been in a Mario spin-off title since Mario Super Sluggers.
Anyway, in terms of the details on some of my roster inputs...
The inclusion of Honey Queen and Pom Pom is so that we have some large-sized female heavyweights to use. Rosalina and Pauline are more considered to be large-sized middleweights (hence the presence of the Cruiser weight category), as they're nowhere near as bulky as the majority of the large-sized characters. Heck, even Waluigi would be considered a large-sized middleweight, especially since he was categorized as a middleweight character in MKDD!! and MKDS.
Sprixie Princess is primarily for 3D World representation, and to add to the amount of featherweights that are not baby characters. She could potentially be replaced by Cappy, but that would mean losing a 3D World representative if Cat Peach gets the boot.
As for Wiggler, it's definitely a peculiar character in that it's medium-sized, despite of its long body. But for balancing reasons, I've ended up putting it in the same weight category as Mario (Medium). Basically it's to avoid the same predicament that happened in MK7 and MK8DX; in MK7, Metal Mario had the same stats as Bowser, Wario, and Honey Queen, but his smaller body frame gave him an unfair advantage over all of them.
In terms of R.O.B., he does warrant another appearance, and would add to the amount of medium-sized heavyweights (he is among the heaviest characters in MKDS) without having to make another metallic version of a character.