[SimplyRecipes][] has a [giblet gravy recipe][SRgravy] that looks interesting. I haven’t used mirepoix in my Thanksgiving gravy before, but that certainly sounds like a worthy addition!
[SimplyRecipes]: http://simplyrecipes.com/
[SRgravy]: http://simplyrecipes.com/recipes/giblet_gravy/
Greg suggested adding some good Spanish Marsala to the gravy. Sounds great!