I'm not sure.
What makes me love Ristar is more on the colourful and cute 2D presentation, not the gameplay (I still prefer Sonic's gameplay).
Changing it in 3D is already making it lost half of its charm for me so I would rather it got 2D sequel but with more playable characters with totally different controls.
If taken in a direction similar to Kirby, I can imagine Ristar in a 3D game. Though given Ristar hasn't had a game since the Mega Drive, I doubt it is in Sega's interest to bring him back.
I'm not going to do anything... but I do have ideas for how a new Ristar game could work. Inspired by Meta-Knight, I can imagine a Ristar OC named Alistar, despite being an anti-hero, still wants to help Ristar.
New "minions" to Kaiser Greedy in mind are Null and Void, twin-villains.
EDIT: I've given it some thought, and maybe a "Raymans Origins" approach would be a more ideal approach for Ristar. Reintroduce him as a reboot with a more animated style like modern Rayman games, but still has that colourful vibrancy as Kirby games.
If I ever feel like attempting a concept demo, I might try it. I can imagine face-designs for Ristar, as well as a brand-new backstory as a demigod. I'm tempted to introduce something of the likes of "
Star Warriors", his father was a legendary hero amongst them and likely the last of the Star Warriors, who the Star Goddess Oruto had a child with. Because of the heroes absence due to Kaiser Greedy's takeover, Oruto awoken her son to save the Valdi System and find his father.