I would much rather like to see a friendship system - maybe like we sort of have with the custom delivery NPCs. Could make daily quests similar to beast tribes where you do partner missions, be that visiting someone or doing a short form version of an existing dungeon or whatever, to raise their closeness. But romance is not a good idea for an MMO.

For one these are established NPCs that need to be cohesively written, so having to figure out how any of them would be in a romantic scenario would cause a lot of backlash from the players regardless of how they are written, because it will likely clash with how most imagine it might be.

Then there is the issue with certain NPC ages, thinking about the twins in particular - but I know there are a ton of others and, as someone mentioned, Lalafell will be an issue due to how intensely childlike they look, regardless of their in-universe standing. Being a better friend to them might work but a romance, especially with how the MC and VISA are currently hounding any form of digital media, would almost certainly get FFXIV put on the blacklist. Same as allowing same gender romances.

Honestly the longer I think about it the more it feels like it would cause drama and anger.
A spin-off game maybe? Dating sims are super popular, it would make more sense to have that kind of thing outside of the mainline game. But the general idea of having some more development for the main cast would be nice.