Lately I've been discussing non-guitar influences and the list is long...
For the lead guitar player, horn players are an obvious choice, because of the single note nature of their instruments. Trumpet players, saxophone players, all the wind instruments have their natural tendencies which are different from the guitar. Learning from the great players on those instruments will expand your playing beyond the fretboard.
For the composer in you, listening to writers that do not begin with a guitar in their hand will lead you towards new chord progressions, new chord voicings, and new rhythms.
Suggested listening: Miles Davis, Cannonball Adderly, Duke Ellington, Charles Mingus.