I think adding foreground plants and a tank focal point like a larger piece of bogwood will fill up the tank in a much better way, and will show of the fish much more, making them more prominent in the set up. I like your tank though, it looks clean and well maintained. Nice photo too.
i see u have 2 boesemani rainbows i dont know if the true colours have come out yet but its is well worth it if you can get them to come out but nice looking tank
Instead of two more boesmani rainbows, go for a small school of thredfin rainbows or a red rainbow. their both really nice and will add a bit more diversity.
If you get more Boesman Rainbows you will see their colur come out and interact with each other. They are a great shoaling fish, really well worth having 5 or more