You can find bags that "layer." See http://wiggys.com/category.cfm?category=6 (and search for FTRSS) for example.

You can also "adjust" your sleeping bag by DIY means:
-You can buy a bivvy sack that can be used as an outer shell for colder nights.
-You can wear long johns or double long johns for exta warmth. Don't forget socks, gloves, a neck gaiter, and of course a hat that covers the ears.
-You can purchase various types of liners. Silk is light but expensive.
-On summer nights, with a full zip sleeping bag, you can totally unzip the bag and use it like a blanket.

Those are just a few ideas; I'm sure others can point out more.