So the most recent Xenologue reveals that Hel, the Norse equivalent of the Devil (although she would be more akin to Hades from Greek mythology) is going to be the next villain in Fire Emblem Heroes. And that makes ridiculous amounts of sense to me.
So why would Hel have beef with us summoners? Well, I would say that it's probably because we keep taking all her stuff. Hel rules over Helhiem, where all the terrible people like Surtr, Garnef, and Valter would go. You know, those guys we shoved into orbs and then brought back from the dead to fight for us for all eternity? It would make sense that Hel would hate us, because we are constantly taking all the people she's supposed to be cosmically responsible for. Surtr even comments that you plucked him from the realm of the dead when you summon him. If you owned a Deli, you'd be pretty mad if someone kept on coming into your shop, shoved your meats and cheeses into Christmas colored balls and then left without even acknowledging your existence. A large amount of the heroes we summon are dead, and if you go with the multiverse vibe that FEH lays down, its entirely possible that all the orbs we summon from contain the souls of all the characters that died while we were playing there respective games. All the times you reloaded because a character died created an alternate timeline, and the orbs are just a multiversal graveyard of sorts. It could turn out that Fire Emblem Heroes goes full Undertale on us.
I would love it if that's the real reason why she's going to be the next villain. It would be so meta and so perfect if we show up in Helhiem and confront her and she just says "stop taking my stuff". I don't expect it to boil down like that, but it would be great.