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.
"missing in the lineup" to me says something that doesn't exist, so not updating existing lenses.
I'm missing:
EFs 11 f/2
EFs 28/30/35 f/1.4/1.6/1.8 (anything small and sharp, Zeiss/Samyang IQ in nifty-fifty-sized case)
EF 85 f/1.4 nonL
EF 100/135 f/1.8L would be nice but i wouldn't afford it
EF 500/600 f/5.6 (with or without IS, just make it light and affordable)
EF 12-24 f/4 (or f/2.8, but it'd cost more)
EF 40-150 f/2.8 (the portrait zoom)
EF 70-140 f/2 (the ultimate portrait zoom)
EF 200-400 1.4x (According to Canon Australia this does not exist "Canon is intending to design such a product. If they are successful a further announcement will follow"
EF 3/400-6/7/800 f/5.6
This is something I'm interested in too... though for me, even a f1.8 would be fine - as a f1.8 would be smaller and cheaper than a f1.4 lens
Incorporating IS is not a deal breaker - though I agree it would be sweet (but again, would add to cost). I agree - needs to have excellent IQ wide open - eg sharp, contrasty, smooth bokeh (9 rounded blades), true ring USM AF and good build quality.
So for me, a Canon 50mm f1.8 III USM ... or a Canon 50mm f1.4 II USM (if that latter not too expensive, and IS an option - again determining weight / cost).