I wouldn't let sorcerers swap out spells. One thing that I'd probably allow, which can be a little dangerous, is to make it easier for sorcerers to pick up spell chains.
I like sorcerers that adhere closely to themes. The problem with staying closely to a given theme is that you're most likely going to come out considerably less versatile that a regular sorcerer or wizard. Consider a sorcerer of red dragon descent, that takes a lot of fire spells, and has a handful of other dragon-related spells (like Fly, Polymorph, and so on). This sorcerer is giving up some powerful spells that don't fit the theme, like invisibility, some powerful enchantment spells, damage spells of other energy types, and so on. It makes for good character building but poor power play.
For a sorcerer like that, I might allow access to an entire chain of spells after they've "bought" two. For example, take Summon Monster I and II, and the rest will be freebies. No feat cost (come on, sorcerers are hurting for feats as it is!).
This option is too powerful for regular sorcerers, though. Then they'd pick spells to maximize the number of chains they can get, which can easily get out of hand.
