They cannot bring Jack back at all. Not only does this ruin the story of Borderlands 2 and what your actions are leading up to that point, but it would mean that the Vault Hunters did all of that for nothing. They lost Roland for nothing and worst of all Maya's death would be even worse than it already was in the third game (That warrants it's own conversation). That's how you kill a series and a franchise. What they should do instead is create a new character and stop character assassinating old ones. The twins would have been fine in execution were it not for Ava in the third game. They had a cool premise. Heck Maya's powers could also have been absorbed and we'd have had a very different boss fight with Tyreen and we still probably could have kicked her tail. But that's not what we are here for, but rather we looking for a way to put Borderlands in the spotlight as a good series again.