Nothing stopping them now aside from still being in high school. I'm almost convinced Marin will get Gojou's hips sore from all the brutal bed breaking she intends to do now that she can proudly say that he's her boyfriend.
Unlike the misogynistic mess you're projecting on him, he's very clear that his jealousy is a him problem and he's moody about it b/c he wants to keep supporting her in what she loves doing.
Hell, this convo was triggered by him coming over to ask what costume he's making her next. And like given how much his moodyness was b/c he thought his feelings were harmful to her levels of unreciprocated insecurity, like it's unclear if he'd still be that level of jealous given her enthusiastic reciprocity.
So...will they do couple cosplay now? Maybe that could be a way for Goujo to get over his jealousy, doing couple cosplay would also be a way to support her in what she loves....Marin could even help her with the doll making... so many possibilities now. The sky is the limit.