I agree with artyH, in possibly a faster wider prime (which means more of a normal view). There are the 24L, 28 f/1.8, 35L, and 35 f/2. There is also the 30mm Sigma but it doesn't fit my film camera so I never looked at it but many people like it (if they got a good copy). Or a macro seems to be missing, so you could pick up a macro lens and try some macro photography (if you think that could be fun for you).
Other than that you are mostly covered, except for super teles.
Otherwise just fill your missing range with a good zoom (though your sig says you have the adequate 18-55 that covers that range) if you feel that is needed.