I'm still in my first year of playing guitar, and what helps me most in progressing my musical education is simply listening to different music. If you're searching for alternative sources of inspiration, you need to get outside your comfort zone. You said you were into late sixties/early seventies classic rock, like Cream, Zeppelin, Hendrix Experience etc. If you want to move beyond that, listen to something completely different! Get yourself some jazz, fusion, acid, post-seventies alternative and prog, country, electronica, baroque, metal, hip-hop, jam blues, Karnatic classical music - whatever, as long as it's different to your usual fare. Even a bit of commercial radio pop will do a lot for your appreciation of song structure and tasteful composition - Robbie Williams is great for this! Listen to a LOT of different music, pay attention to the musical ideas that show through, and these ideas will manifest themselves in your playing.