Lets get the theory ball rolling again, how and why do you think the MC will join Ella for a short time, or will he join her at all?
I ask this because there has been a lot of foreshadowing that Ella is not dead and intends to nab the MC, and we've seen some talks between higher ups at hero about whether they can trust the MC.
personally i think either
a) The MC's gonna be in a major fight (maybe with sin?) where he wins but he's weakened and that's when Ella reveals she's still alive and disappears with the MC
b) The MC might find out something shady at HERO that personally relates to him (maybe they screw him over or he finds out they were behind his mothers death?) and he's forced to hide out with Ella
I also wonder if the MC get nabbed by Ella will we get an arc where we learn about how she became superhuman, what happened at the massacre, why she chose us to be her chosen, how she knew us before we met, you know stuff like that
To be honest, I don't think it'll have anything to do with HERO, specifically. I believe that going with Ella is going to be a part of the plot like joining HERO. It won't matter if the MC is for or against HERO or Ella. You'll be forced to join Ella regardless. As for the cause, it's likely that the 3rd and 4th are going to intervene in some manner to command the MC go with her. We all know Ella isn't dead but she won't be able to escape HERO and take the MC with her at the same time. At least not by herself.
Originally, I thought maybe Ella could forcibly take the MC but he's too strong now and depending on how far he develops during this arc, Ella won't be able to make him come. That isn't to say Ella can't beat the MC. It's more that she won't be able to subdue him as easily as she could in the past. There's also the fact that MC has built up super strong allies that would prevent it from happening. That's why I feel like our monster parent is going to be involved. That's the only way we can be made to joined Ella.
From a plot perspective, it makes a lot of sense for the Apostles to make us join our "Cousin/Sis" kin since they're supposedly on their way. Naturally, the MC will have to join them as their child, no matter what. It also wouldn't conflict with players who like HERO and don't like Ella at all, since it's being done via an outside party of the two. Even for an Ella simp MC, he wouldn't just up and leave everyone for her, so the driving force behind it all would possibly be something well out of his own control. Regardless of what that is, it's not at all because the MC decided to join Ella. Ella pretty much confirmed that he has no say in it at all and that it'll inevitably happen.