I posted this theory in a comment somewhere so sorry if you're reading it again, but anyway, I think that Lucy will be the one to completely defeat the nine demon gates (not defeat them individually but make sure they can't come back after they're defeated or anything) because the group as a whole is the nine demon GATES and her magic revolves around gates and keys (celestial spirit magic), I'm guessing that after one of the demon gates is defeated they will turn into a key (or Lucy will have to turn them into a key herself with a different kind of celestial spirit magic) and then she will make a contract or something with the demon (maybe not make a contract but be in control of the demon so that she stops them from coming back) and it would also be a way for yukino to come back because maybe Lucy isn't powerful enough on her own to turn the demons into keys
Another theory I have is that maybe the demons were once celestial spirit mages that zeref twisted and turned into evil creatures that obeyed his control, so Lucy and/or yukino have to turn them back into celestial spirits.
Let me know what you think of this theory but don't be too rude if you think it's full of shit xD thank you for reading :)
Edit: also maybe celestial spirit magic isn't actually celestial spirit magic but a magic that allows the user to pull something out of another dimension with an object from that dimension, (ie. the celestial spirits with the zodiac keys.) and so far the only things like that have been the celestial spirit keys and so the magic was wrongly named celestial spirit magic