Basically, Noctis is a summoner, and the starscrouge manifests as a being that spreads its sickness to areas of high concentration around Eos. He and his guardians are traveling the world to acquire each summon/astral before reaching the final battleground. Or you could just put the cast in Spira and build your own world, I don't mind!

If the author would rather not include a pairing, that's fine too. But if you do, I don't have a preference! I just love the world of FFX and its story so I think combined with this one it'd be super cool.