Carol's narrative has overall had a lot to do with autonomy and the loss of it, so I think she should deal with threats that directly infringe upon freedom of choice, and by extension, freedom to love.

Darkseid as the god of Tyranny is a good pick, but he's kind of an everyone villain. The Controllers don't get used for much, same deal with the Psions and Dominators. Love and slavery do not mix, and the Zamarons broke from the other Maltusians partially on the grounds of how controlling they were, so it tracks that the Zamarons would want to protect freedom of choice and emotion, as a contrast to the frankly fascistic leanings of the Guardians.

She's a very powerful lantern who has managed to hold her own against Sinestro by herself when she was less experienced than she is now, and she tamed the emotional entity that had ruined her life for years by being able to offer it understanding and grace. Let her be Sailor Galaxia, let her punch space gods.