I wouldn't mind if it was like a quest series in the patches of the final expansion before the game goes into maintenance mode, but I don't think there's any chance of it happening before then.

Partially because it would limit their story options. Yeah, characters already rarely die as is (Y'shtola almost seems immortal at this point with everything that's happened to her), but if there was a romance system, it would further limit who can die since people would probably get pissed if they romance someone, only for that character to die later.

And beyond that, how would they choose who can be romance-able? The game has a ton of characters with varying levels of relevance. Would it be just the scions? The scions plus country leaders? Do we include class trainers too? Are you wanting personalized romance subplots for each character, or are you okay with a Skyrim-esque system of "Hey, you wanna get married?" "Ok bby", because the more options you have available, the less effort that will go into each one.

And then after that, does each character who is romance-able change their dialogue around you or do they act the same way? Does Alisaie still act like a schoolgirl with a crush, or does her dialogue totally change whenever that comes up? I remember it bugged me in Persona 5 where you romance a member of your team, but they continue to act like platonic friends in nearly all dialogue, even a situation that might be relevant comes up (Like if you romance Ann before Hawaii, you still get the scene where you, Ann and Ryuji are asking each other what they are wanting in a date partner, and none of the dialogue changes from if you were single or with someone else), and that kinda bugged me and I would bet it would bug others too.

And then there's just the added work of changing dialogue all over the place if you go the "more effort" route, but with as much text as XIV has, that starts to add up.