Who said there needs to be one? I'd be all for them working side-by-side towards a common goal, and if Hickman is ditching the traditional team method then there's no need for an allocated leader. If anything, their shared experiences should allow them to put their differences aside and come together for the sake of all mutants, especially after the events of Rosenberg's Uncanny X-Men.
But isn't that what would make it interesting? I think having them in a shared space would make for plenty of drama. And who knows how Sean would react given his current state anyway?