Super hot take but Akira is, like, the best character. The fact that she is an asshole who is slowly becoming better is, like, good. Character arc and stuff, you know? People are malding for no reason and it is highly weird. But yeah, calling it now, Akira will be the one to initiate the whole poly thing, which is a fucking huge brain move on mangaka's part.
Aside from being interesting and unexpected, it also makes sense given the fact that Akira, right now, is the person who knows the most about everyone. She knows that Mayuki is into her, that Mayuki was kissing with Rin to ask her out and she probably can tell that Rin is feeling something for Mayuki. Like, she only needs to confirm that Rin has feelings for Mayuki and, either through Rin or Mayuki or just the internet and herself, realize that poly is, like, a thing. From that point, it is a straight(not really) road to pogcity.