I always do nicely with one good strong gourami as the main fish in my Thai tank - had a blue and a pearl. Will be getting another pearl soon. A nice school of rasboras and some SAEs really rounds it out. Though you could easily go with loaches if you're not being as regionally specific. (They're from India.)
Your tank made me nostalgic. My 20g long was once in its opening stages and was sparse like that with just some moss and java ferns under low light.
If you want some suggestions, i feel obliged to give them in light of our similar paths.
The more simple things you can do:
Try training the moss onto the driftwood. And get more driftwood - one or two pieces tend to look unnatural and awkward. Once you have three or more, you can really start getting a naturalized look while also hiding the equipment more effectively. To train the moss, all you need to do is tie it to the wood with some black thread. Nature will do the rest and provide you with that nice, well aged driftwood feel.
Try doing a little terracing with some more rocks like the nice one you already have. It will also enhance the naturalization effect, giving the layout an appearance of a naturally occurring creek side pool. Biotopes benefit immeasurably from this simple touch.
Consider red substrate and rocks - a lot of Asian backwaters have iron rich sediment making up their bottoms. You can simulate this with a good rusty red hue.
Consider Malaysian trumpet snails. Some people dislike snails, and though I disagree, to each his own. But MTS are part of this complete ecosystem - they help break down detritus, and keep your substrate from compacting. The downside is that they really can breed to the point of infestation. (Though I'm not at all bothered by it - I like them.)
More advanced options include:
Using peat filtration. To truly simulate the native habitat of the gold/blue or pearl gourami, the harlequin rasbora, etc the water should be humic. At first I was skeptical about this but once I started doing peat filtration I saw colors and energy in my fish that I never knew were possible. Plus I personally like the golden amber hue of the water - it really enhances the naturalized effect. Plus it is a moderate antiseptic - staving off algae and bad bacteria.
Diffuse the flow from the filter. These habitats are very still, sometimes even to the point of being stagnant. (This is why anabantids like the gourami have evolved to possess the ability to breathe air.) These fish will appreciate a break from strong currents, and again you will have a more natural regiontope.
