95%. With how close they already are having them become romantic wouldn't change much for how they interact and support each other, and they have ways to explore a romantic relationship between them dramatically without resorting to basic cheap shit like misunderstandings or cheating. Star's status as a princess and the Blood Moon are both open-ended plot lines to be explored when they end up together.
The show is (probably) halfway over, and even though a whole hell of a lot can still change I feel like it's definitely too late to introduce another serious love interest for either of them; Jackie and Marco's relationship was built up for a whole season and a half and still hasn't been resolved. At the same time the fact that Star has already confessed makes it very unlikely that they can stretch the current love triangle situation for more than another season.
Even if they don't explicitly start dating I expect Marco will start to return Star's feelings before the end of Season 3, maybe as soon as the mid-season finale.