A couple of things said by Sarah in her livestream:
The upcoming novella will be about Mor, and it could come out around September this year.
The next book will be about Azriel and Elain, it will probably come out in 2022 or 2023.
The final book will be multi couple (like Kingdom of Ash was) this might mean Feysand, Nessian, Azriel and Elain.
Amren will not get a novella since her story finished and she has her HEA (became full high fae and is together with Varian)
She wants to write more books in the acotar world about the other courts.