Hi WesG,
For what purpose do you mostly use your camera? (landscape, buildings, close shots, macros, etc.) Where do you shoot mostly? (Outside, sunshine, low light, etc) How much money you plan to spend on lenses?
I have a C20D and my all-purpose lens is a Tamron 28-75 f2.8 Macro, which is a pretty fast and reasonably weighted lens. For closer shots I use a Canon 80-200 f4.0L, which is a great value for an L lens. I certainly want to have a wide-angle one and I am waiting to test a Tokina 10-17 f2.8. Besides that both Sigma 12-24 and Canon 10-22 produced beautiful photos.
Does your 70-300 IS a DO (Diffractive Optical) lens? If you are happy with it, I would not buy another one. The range on a dSLR and IS should give you enough room. Again, if you want to burn some money, a Canon L lens is a good investment (you can also use it on full frame Canon camera if you decide to change the current one.)
Seeing your lens collection, I judge you are probably more informed on Macro lenses than I am. So, let's wait for someone else's comment.
Cheers,
Bandee