Maybe Something traditional & nautical themed to go with the mermaid? Could help tie together with an eventual gap filling plan if you are thinking of gap filling eventually. I can’t tell exactly, but it appears like the elbow is under the mermaid. So, some sort of undersea thing...?
I’d think bigger now, just to be proactive. You don’t have to actually do it, but you’ll at least not paint yourself into a corner if you do decide to eventually build to a full back piece. You never know. And that’s some high value real estate.
I would, in the process of refreshing what you've already got, embelish and expand what is already there. You could join them in that process.
You would need to find the right artist for that endeavor, and we can help with that. Where do you live?