I get this way of thinking but I don't know if a character's current popularity is the only key. I would also think about whether they have a central role in any really popular critically acclaimed Marvel storylines and sometimes even that doesn't matter.
I mean for the life of me, even all these years later, I cannot understand how Marvel built a successful franchise out of Guardians of the Galaxy. I will never say a character can't work after that. And once the MCU consists of film, Disney+ and Hulu shows they'll need looooots of content