HKLovingIT ... I like your sig line.
Sleeping bags ... hmmm ... well, I for one do believe that your sleeping bag can be too warm on a hot summer night.
Not sure that there is a single bag that will do it all. Sorry--I don't have a loophole for you on getting around using the right bag for the particular conditions--especially if we are talking a wide range of temps.
It really depends on how you will use your sleeping bag--when, where, etc. You mention temps as cold as in the teens. Given that, and assuming you want just one bag--I'd go with a bag rated for 0 degrees. Then, I'd spring for a silk sleeping bag liner. They're not expensive. A silky liner can add additional protection within your bag ... or you can use it by itself on top of your bag on a warm night.