In the development of great solos, to me it makes sense to think from a songwriting perspective. Think like a vocalist. I read an article with Tre Anastasio (Phish) a few years ago where he talked about listening to Billie Holiday (jazz vocalist) and how he would learn her vocal lines on guitar. Great soloists to me are also good songwriters and can sing as well. Thinking from a a vocalists' mind frame I feel really helps your solos to become much more memorable and melodic. I know Slash doesn't sing, but he has been in countless great rock bands over the decades and I am certain that melodic sense is in his playing.

Try learning from vocal melodies from songs you like and see what happens. Good luck! :D