You might consider Boraras - either Boraras Briggitae (Chili Rasboras), or Boraras Maculatus. They grow to barely 1/2" to 3/4" max., and you could have up to 2 dozen in that size tank. Lovely little fish - check them out.
I considered shelldwellers. Mudskippers would be awesome and I've always wanted to set a tank up for them, and will one day. Just not at the moment. I actually have this 20L established and planted already. Just waiting for fish.Shelldwellers are on my short list of tanks to set up. Mudskippers would be different as well but would require setting up a brackish vivarium. Might not be the direction you're interested in going, but those are two options I would consider.