Are you asking which lenses we would like to have, but Canon does not make? or is it which lenses we are missing from the canon catalogue?
I've a pretty broad selection, so I'm not really wanting any of the existing lenses enough to actually pay for them.
I'd like to have a more light sensitive body, like the 1D X.
In terms of missing lenses in Canon's lineup, I don't see any gaping holes in the focal lengths, they are well overlapped by the many zooms and primes. Some updates of the older primes might be in order.
If there is a weakness, its in ultra wide prime lenses, I'd consider replacing my Tokina 17mm f/3.5 prime with a 17mm f/1.8 or f/2 or even f/2.8 prime. The TS-E 17mm is a fantastic lens, but my vision does not let me manually focus.