it follows that the characters the fandom is most likely to latch onto are going to be the most interesting ones

I'm chuckling, not at you but because DARCY. The 'fandom bicycle', because she's bland, cute and adorable and fun and lightness and rainbows made flesh et al, vaguely fleshed out just enough for fans to use her as a self-insert meat suit into any and all fandoms. I'm sorry but your argument doesn't hold water in light of Darcy, the fandom cockroach who will outlast popularity of Marvel itself.