Of course he is going to do something, he is already planning to make chimeras with omegas.
Magneto and Xavier believe they are smart enough to use Sinister and not the other way around. As we already know, Magneto and Xavier think they are smarter than they normally are.
Captain Britain for Other World, Namorita for Atlantis or Selene for just being evil.
That applies to half of the Quiet Council, and the fact they haven't done anything off the books (yet) isn't a guarantee they ever will. They're all time bombs waiting to go off, and even when they do they'll stay there - see Xavier not throwing Apocalypse to the wolves after what he did in Excalibur. That precedent gives Sinister lots of space to be evil and get away with it.
I think Cannonball could be a great choice for the Council. He represents a younger ideology, has served as a teamleader on many occasions, and has been taught by Magneto and Xavier both (and Cable).
He doesnt live on Krakoa bc he has a wife and family in another galaxy. I dont think he's too active on Krakoa to serve in such a high government position. If you wanted someone to fill that younger ideology role, Dani would be a perfect fit as she also was mentored by Magneto and Xavier,
I think both Sunspot and Dani would be excellent options. But Sunspot is enjoying space adventures so, Dani.
Watch it end up being Glob Herman.
While I love me some 'Berto, he's not all that serious, and I think Dani would be a better choice. She also has a fairly unique perspective as someone who has spent a fair amount of time as an 'ex-mutant', getting by without her powers, and would give a voice on the council for the potentially millions of other 'ex-mutants' (or 'ex-ex-mutants' who were depowered, but have since been repowered, and are going through that life-adjustment all over again) out there, who are currently voice-less.
Karma would be another one I'd consider for a leadership role, but, as of yet, hasn't really gotten a chance to shine.
(Pretty much any of the original five New Mutants seem like they'd be better choices than Ilyanna, IMO, whose only real leadership chops appear to be 'artists like to draw her with her big sword and tiny outfit').
That Roberto, while hyper-competent and cool, is not the flightier and more whimsical Roberto we've seen more recently in New Mutants. I like both interpretations (and wouldn't mind a more consistent fun, yet still competent, version closer to the middle of those two more extreme interpretations), but don't feel like the present interpretation would be quite as well-suited to a leadership role as Dani or Karma or, perhaps, Sam.
Ewing's Roberto was based on Hickman's own from the Avengers, Roberto took over AIM with Hickman. There's no reason he can't go back to that characterisation other then Hickman choosing to. Him being on the Quiet Council would be a natural evolution of his experiences, rather than backsliding into being a follower. Sam would be great but I'm not sure he'd do it after the Crucible, however, I'd like to see him clashing with the Council.
I agree that Dani would be a fantastic option, but I remember that Magneto, Storm and Xavier also lost their powers for quite some time.