Your slate wall is awesome! great job with it. I really think that should be your main focus on the tank. You could also consider a small piece of driftwood. I could see this tank with mosses and java fern on the slate wall with some sort of larger stemed plant (like an anubais) on the left side and a small piece of driftwood with some mosses/java fern on it on the right side. The rest of the plants would go in like you were thinking framing up the slate wall.
I wouldn't consider this the ugliest tank at all. It's got a lot of potential and you seem to have the knowledge to get it done.