Someone asked to write a fic based on my idea and I’m unsure of what to reply

Hi! I posted an idea for a fic on tumblr (something I plan on writing once I have more time) and someone commented and asked if they could write it. My first instinct was to say “sure go ahead” but now I’m thinking about it and I’m not sure. I would be more open to saying yes if this was someone I was mutuals with, but we don’t follow each other and have never interacted (I also did a search for the ship on their blog and didn’t find anything, but I saw they follow a couple of authors from the same fandom). I don’t own the idea, of course, but I did come up with the premise and it’s something I really want to write. I would like to know your thoughts and what you would do in this case. Really appreciate any advice!