Before you can develop a theme, you have to have a theme, try putting the guitar down, forget about the solo and guitaristic devices, what is your melody? Can you hum it or sing it, does it sound complete, what does it need? Does it resolve or just wander off into the unknown, think of the musical idea as like writing a complete sentence, until you have a complete musical sentence, don't pick up the guitar. I kind of anaylized what I do, and normally I pick up the guitar and play the basic chords out first, I realized that I do this because I already have the melody in my head and am just plugging in the chords, give it a try and don't worry if it sounds simple at first try.......