well... I might have been wrong. they've reached the point of killing off too many people now. Getting rid of Rick was huge in of itself, but doubling down on Maggie (with literally zero explanation so far) might just be the nail in this undead's coffin. No one gives a crap about Hilltop without Maggie. I mean... seriously... who gives a crap about Tara? And back at Alexandria, who cares about Rosita or Father One Eye or Mullet-head?
And with all the other characters that people do still care about (Daryl, Michonne, Carol) they're all spread out.. at least they have been the last three episodes. Hopefully the new bad guys bring them all together, but even if they do, that trio still isn't interesting enough to hold the show together, especially with what they're probably about to do with King Potty-Mouth.
Tara, Rosita, Gabriel, and Eugene have all been in about 80-90 episodes. I'm sure a lot of viewers care about them in varying combinations. They're probably not invested in them as series leads because they've never been leads, but minor characters are not without consequence, especially in show that cycles through minor characters much more quickly than leads.
IMO Eugene and Rosita could conceivably level up, though probably not to top billing. Eugene's character would have to broaden and mellow a bit, with his awkwardness relied less upon for laughs. Rosita is in a lot of ways still a utility player. We know some of her backstory but not too much. Subplots where she's a focus haven't been so memorable, but she has the tools for survival in this world. (I do think the relationship with Gabriel feels forced, though, especially "right after" his relationship with Jadis--which I'm assuming was a big deal because he's a priest.)
Other candidates: I liked Jesus's character and was happy to see more of him, but the show would do well to recognize which of its characters have intrigue that should be maintained rather than diluted in a central role. By contrast I think Aaron might be very suited for a leadership role. The hard decisions he would have to make would be relatable as hard decisions, and wouldn't require the sometimes-manufactured agony that badass characters such as Rick, Michonne, and Morgan have needed to be three-dimensional.