Hey! I'm fairly new to this hobby, but I figured I'd put in my two cents. I really love the layout! I don't know if I'd change much to be honest. The open look is really nice and the bunches of plants around the rock work really well.
If you wanted you could try a tall spindly type plant with the others to give it more height. Or do a carpet plant. I like the colour theme, but adding something red might be interesting too.
It's beautiful as it is, though, imo.