Dozens of threads pop up everyday for stocking advice. I’d likely try googling something along the lines of “20 gal aquarium stocking ideas”. It should turn up some decent lists.

Beyond that what do you like? Schooling fish? A big predator? Do you like inverts? Hoping for cichlids with personality? A betta? There’s thousands of fish that can go in a 20gal. With nothing specific in mind browse other people stocking lists or aquarium fish sites for inspiration.

personally I’d look at Badis, killis or gudgeons as a centerpiece. You could do a colony of any of them. They stay pretty small but are super colorful. Badis are related to bettas and gourami with tons of personality. Killis are some of the prettiest freshwater fish and some are fairly exotic and rare. Gudgeons are really colorful and peaceful (my pair swim with the guppies in my angel tank).

Shrimp are low bioload and help clean. Snails are higher bioload but typically eat more algae.

Aqadvisor has a decent calculator to play with stocking and tank specs.