What to do if someone wants to make a fic for my AU?
So I have an AU on tumblr that for me, something I would be making on which is a fic. Especially when I found out people wanted to read it. Anyways, pass forward months, someone suddenly uploaded in the main tag of the fandom searching for my AU, citing they wanted to find it cause they wanted to write something for it. I read it and realised it was my AU thus commented it and they confirmed it was that.
Now the problem is, I don't know what I feel with someone making a fic for this AU.
First because the person stated in their post they were trying to find a prompt and my AU is not a prompt, its something I'm actively working on to publish myself. I guess they thought it was.
Two, I'm honestly flattered too (like I wanna do art for them cause I do both fanfic and art) but the thing is they haven't asked me one bit if they could use my AU which puts me in a weird spot if I would allow it or not. I'm not even sure they would credit me.
Third, I have the stance that 2 fics for the same concept is the more the merrier but I don't want their story to clash with mine even if they want to use the AU. Plus, i kinda have more planned for that Au.
Fourth, if they really want to make my AU, I guess I could just give them my plans? I haven't really started yet on mine other than notes, but I'm not really confident in my writing so if I opt to go message them to give them my notes and guide them on how I want to make this fanfic, I wouldn't be making my side of the story anymore as it becomes a collaborative. The thing though is that I liked this Au and fear that they wouldn't do justice to it if they take creative liberties. I'm aware everyone has a different perception of their beloved characters but I'm very happy with mine, cause OOC or not, as long as you recognise that they can do it then they can. I also kind of fear they would suddenly leave off the story abandoned because I'm not planning a one-shot. (Its actively a series in my head)
What should be my response? Considering they're not really reaching out to me other than to message under my comment that it was my Au. I feel conflicted both as someone who likes to read things and also to make things for my fandom.