promoting the existing one is a good practice. However, like @payothemes said, you should make both. I’ll go further by saying that extending the template should be your priority. To be honest, like i said before, even with strong advertising, you wouldn’t sell much more. Look at people that made the HTML ICO/Crypto theme, it has absolutely crazy sales since it’s release in march 2018.
They chose a niche wich is currently top rated in the current sales trends (this is only my own opinion). Cryptocurrency, blockchain and bitcoin are at the top, but this might change brutally.
Always keep an eye on the market trends to decide the kind of item you are going to make, and believe me, this is the key. (or at least, one of them)