Not sure if this is a joke or...
(like seriously though, what would the story even be for this one? You already have an established harem, the game's big bad is essentially gone, and while there's still the grimoire to deal with I feel like that issue could be solved in a one-shot epilogue-esque game and not a full-blow sequel.
Then again, I can see a story hook where
the remnants of the Sons of Astaroth manage to track down and capture the OIAL MC and successfully steal the grimoire and his powers but OIAL MC escapes and now he has to travel around the world trying to reunite with the rest of the cast who're likely hiding from the Sons, which gives him the excuse to seek help from a new cast who're going to add into his ever-growing harem until ultimately he reunites with the main OIAL girls and prevent the return of Astaroth/completely banish Astaroth from our world or something.
And personally, I'm not sure if I'm a fan of that since it feels like the story is being stretched out for no apparent reason. But then again that's just my speculation on how things would possibly go down. Another scenario would be OIAL MC messes up and the girls leave him and he has to start from square one trying to win them back...but that would be an even worse scenario I feel.)