What about hatchet fish to fill in the upper regions? They tend to swim higher in the water column than rasboras or barbs, which may help balance out scape
As far as other suggestions go, stick to bold fish that don't need a lot of plants/coverage to feel safe. Barbs, rasboras, and danios are all good options. A single dwarf gourami might work as a centerpiece fish if you don't want all schoolers, and will also tend to stay in the upper range of the tank. Avoid loaches or cory cats if you want groundcover plants, as they will tend to uproot everything while hunting for food
Shrimp are much better bottom level options.