I started with a 20 long too! mine didn't look half as nice as yours does though... had nice white clown puke with 3 sword plants from petsmart. I guess you can say I've grown up since then... Your tank looks great through. Keep up the good work.
Damn heaters and their awkward sizes. If you put it in the back left corner horizontal behind the stems it might make your stems look a little fuller because it would push them forward a bit. The plants will find their way around it. It does look like a good setup as is though because with your outflow blowing on the heater you want get cold spots in the tank when the heater is on. Not that it matters a ton. Good looking tank though, the 20l has so much variety of planting styles. Probably my favorite tank size so far. I want a 40b pretty badly! Too bad stands for 40b are limited outside diy. :0
Some Anubias Nana petite would look great in the right side of the driftwood.