For me, it's mainly obscure killifish (Aphyosemion coeleste, ocellatum, pamaense, mimbon and species not yet found). I have or have had the killies on Crispii's list, and they are fine ones.
I'd like some Enigmatochromis lucanusii and Pelvicachromis kribensis dwarf cichlids ("Kribs" are actually P pulcher, and the real kribensis is a different fish - an ancient identification mistake).
Aspidoras catfish would be neat to have again.
The mainstream, corporate purchasing hobby is very limted in what it has to offer, compared to what nature has out there. I've had a at least 30 tanks for 30 years, and have been in the hobby for 55 years, so I have kept a lot of different species. With the exception of A. pamaense, every fish up there is something I've already kept. but stopped keeping. It's a list based on infinished business. Sometimes it's not the novelty of a fish but something you want to learn more about.