There are many wonderful multifandom exchanges throughout the year that overlap in niche-ness both with each other as well as Not Prime Time. While many of these exchanges are for small fandoms, most of them feature medium sized fandoms as well (or at least fandoms that NPT accepts for its eligibility).
The Exchange at Fic Corner: fic_corner
. Children's and YA books and short stories from picture books to edgy teen novel.
. Fanfiction exchange for fans of femslash.
Jukebox Fanworks Exchange: jukebox_fest
. Song-based fanworks.
. Multimedia fanwork exchange for rare chromatic source fandoms.
Parallels Fanworks Exchange: parallelsfic
. Rare Asian fandoms.
Rare Characters Fanworks Exchange: rarecharacters
. Multi-fandom exchange community for underrated characters.
Rare Pair Exchange: rarepairfest
. Rare pairs in ALL fandoms, canon or non-canon pairings.
Rare Ship Swap: shipswap
. A community for the exchange of fics dedicated to rare ships.
Rare Women Fic Exchange: rarewomen
. Rare women, from big and small fandoms.
. Rare fandoms exchange.