Thank you everyone for your replies. You've definitely given me a lot to think about (which is what I was hoping for). I think I'm leaning towards a wide angle prime right now. As it is, I have triple coverage in the 50-60 range (15-85, 60mm macro, and 55-250)
These are all very different lenses. There's nothing from with having some overlap, what you want to avoid is duplicating function.
You wouldn't be duplicating function by getting a fast lens.
So, that's why I'm thinking a fast wide angle prime would be the best addition.
28mm is a "normal length" lens on APS-C. You won't get nearly as shallow dof as you would with the 50, but the fast aperture will help in low light.