Why dont you do it yourself?

I've made some riddle-quests for my FC before - its not as neat as actual quests, but you should sent her on a little treasure-hunt, while employing friends and FC-mates for an afternoon to function as questgivers, standing in specific locations, giving her little tasks and riddle thats ultimately lead her to the place where he is waiting.
I doubt that the devs will add something like this - but specially if its only for a day you could totally set it up yourself! Starting with a little mogmail letter, leading her to the first "NPC", who can give her some little quest-task (gather something, bringing back monster drops, craft something, use a certain emote... basically all the things our usual quest-NPCs ask of us aswell). They could even give out quest-rewards - flowers, maybe or perhaps there is some expensive glamour-gear she had her eyes on...? Mounts, minions, orchestrion scrolls are all options aswell! And either have them tell her excatly where to go next - or give her a little, maybe even personalised riddle ("The first place I ever had to raise you" etc.).

I doubt the devs have the ressources to create personalised quests for such events - but making this yourself, with the help of all the friends the couple made during their time in FFXIV, is probably even a lot more meanigful. Just my personal opnion, but a proposal shouldnt be something that one "orders" from a company, but something someone creates themself, puts the required thought and work in - instead of asking someone like the dev-team "Hey, could you write a sidequest thats basically me proposing to my girlfriend?"