Dragon Magazine actually had an article in early 2001 that had some good example of this.
Some other ideas incude bargains with other-planar creatures. Imagine that your first level sorcerer is REALLY nothing more than a commonoer who yearned for a path out of poverty, and so literally cut a deal with the Devil (or Asmodeus, or Baalzebul, or WHOever) to gain sorcerous powers?!?!?
What kind of roleplaying would come out of such a scenario?
Also, including birth by other creatures than dragons - imagine you have the blood of a Solar or Planetar running in your family! What would this discovery do for a Chaotic Neutral or an evil sorcerer, even?
Finally, Sorcerers could be the D&D world's version of mutants - the "X" gene came to life in their family, and they are the start of a whole new breed of super-men and super-women, exploring the depths of their powers.
There's a whole world for sorcerous beginning beside just the blood of dragons.