Struggling to Come up with sleeve concept

This is a common problem, one that honestly you should discuss with your artist. My recommendation is to come up with a style you like (fine line, traditional, realism, etc) then come up with the thing you'd like to convey with the tattoo (a message, a specific visual, a feeling, etc). Finally then look on this site for artists (either in your area or based on the style you like). Don't look for exactly what you want, but make sure you like the artist's capabilities, their art, their work. If you do then reach out and begin a conversation with them. Openess and a some freedom to do art is always a favorite combo for an artist.