For a Nature Aquarium style
HC for the ground cover
Staurogyne repens and moss covered rocks intermixed along the inner edge of the wood
Blyxa japonica for the left front and along the lowest branch of the left wood group (this is to hide the bottom parts of the stems behind)
Anubias nana 'Petite' on the wood
A mix of your preferred stemplants in the rest of the space.
For just a general "planted aquarium with the good stuff" with species chosen from Tropica's 1-2 Grow tissue culture series that I remember the store I worked at was able to get. They don't necessarily need to be the Tropica stuff it was just a good list to go by.
Well defined groups of:
Pogostemon helferi "Downoi"
Alternanthera reineckii 'Mini' (way overused these days, but is a good size for a 20L)
Weeping moss for the wood, or a bunch of different Bucephalandra, up to you.
For my next trick; tell me what fish you want to keep and I'll do a plant list for them.