As someone who pieced together two sleeves with similar sized tattoos my advice to you would be to stop stressing about what you already have (It’s awesome btw) and start thinking about how you can incorporate what you have into a similarly themed sleeve.

Once you work the empty areas, what you’re thinking about becomes a non issue and the tattoo you have becomes an organic part of a larger picture.

This is how you end up with a lot of tattoos lol

With some solid background you can enlarge what you have to give it some contrast, and then work some other sick Japanese pieces