Aaaa, sorry Anon, I didn't notice it sooner ;n;
I'd say Arranged Marriage AU. It may sound weird but for me it has an appeal of 'what if'. What if Rea wasn't from our world and wasn't a Sage but a witch (and noble) from Eastern Country. How their story would turn.
Funny enough, I made Merfolk AU for Mermay; also have Vampire AU but I'm not the greatest fan of these themes in general. To be fair, Arranged Marriage isn't my fave either but it work for them in these circumstances.