In this book you might find both X Reader situations and Character X Character, although it really depends from the ship. Feel free to ask whatever you please, I do not assure I'll accept it though. Some ships I refuse beforehand are DouShino (Douma and Shinobu), or any ship involving minor Nezuko, unless it's platonic.
As for other things, you may ask pretty much everything, from angest, to smut (which might take longer to write, since I'm not used to it anymore), to even the cutest fluff.
List of characters is up to date, it may vary if I get other requests, as for example Kaigaku.
Have a nice day!