I am smoking a dozen St. Louis style spares this coming weekend for a family get together. I have tried a couple of different rubs with good success but I am itching for something new. So far I have tried:

Meat Heads Magic Dust

Another favorite was Steven Raichlen's 5-4-3-2-1 rub.
5 parts paprika
4 parts brown sugar
3 parts salt
2 parts black pepper
1 part dry mustard

I am looking for something with just a little more heat and good balance of sweet and savory, possibly with a hint of sour/citrus.

So where should I start looking? What are your suggestions?

Neither paprika nor dry mustard adds much to rubs used for long cooking in dry heat (Raichlen should know better). Paprika loses what little flavor it has, which isn't much in the first place. Dry mustard needs liquid to activate its flavor. Though the moisture that exudes can (maybe) activate its flavor, the flavor is destroyed by the heat of cooking so the addition is useless.

See this post (and the recipe link in it), and the posts below for subs for the paprika.