I don't know if you've read the ending of X-Termination, if you haven't be warned:

SPOILERS ABOUT X-TERMINATION ENDING, DON'T CONTINUE READING IF YOU DON'T WANT TO SPOIL THE END OF THE CROSSOVER.

Ok, after warning, here's what happens at the end of the crossover: AoA Nightcrawler after seeing how Kurt Wagonner, a child version of NC from an alternative reality (in Dazzler's interdimensional team) sacrifices himself, realizes how much good his powers could do for the world and in a crucial moment he decides to follow the good ways of that child NC and sacrifices himself too by transporting the generator of a portal to the 616 world to the AoA world where the other door were and where some dimension destroyers were destroying that dimension to close the portal and avoid them from destroying the other realities too (like making a door direct you to the same room you're in).

I have to say that I've never liked AoA NC too much (Kurt along with Cyclops is my favorite character so an alternate version isn't a good enough replacement for me) and I've always seem him just as the example of Kurt powers potential without morals. But his ending was good, he decided to do good and ended up sacrificing himself (which makes 3 Kurts dying in self-sacrifice, 2 of them in the same event).

Now while the death was good, I think it could've been a perfect way to bring NC back, I was following the event without following any of the 3 series involved mostly because I was interested in NC possible return, given how much you can play with reality jumps and how 2 NCs were in the event so it was heavily related. When AoA NC sacrifices himself something could have happened, like 616 Kurt from Heaven strengthen him for finding good will in his heart and something as paradoxycal as bringing a reality portal exit to the reality where the other exit was creating an anomaly and in the process Kurt spirit being fused into the other Kurt body and making a return but not retaining some of his memories, specially the ones from being in Heaven or just small bits of it, remembering that it was an amazing experience but without making him crazy for the big jump from Heaven to earth.

IDK, that was just the first thing out of my mind, but I'm sure there would've been many ways to bring him back with all the ingredients in the event but at the end we basically have every single famous Nightcrawler killed.

As a religious character NC (whose spirit has already made an aparition stating that he was in Heaven and that they had the best beer in Wolverine's main series) it's very tricky to bring him back, and I think X-Termination had good ingredients for that with something like what I said so he could come back in an anomaly event so he would only have fragments of memories of Heaven so he wasn't obsesed with the drastic change from there to earth and stay the same good character we remember.

Now I guess I'll wait for the future Azazel and the Bamfs story in WatXM as that one (his father related to heaven and hell dimensions, multiple toon/chiby copies of him...) has potential for his return too... maybe all other NCs were killed to focus on the real NC when he returns?