If you can play the guitar really well, you could try branching off onto other instruments, so that you can eventually write for more than one type of instrument in the future.
You "can" play any note over any chord, it depends on what you're trying to compose. Any note added to a chord makes it a different chord. It depends on the next chord you pick in the song where you usually can go with a melody, etc. Share some more about what you're trying to accomplish.