There are already too many existing lenses on my wishlist to be too bothered about new releases, but if I were to vote for one, it would be an IS version of the 180mm f/3.5L Macro - as some have suggested elsewhere, maybe taking it to 200mm.
I'm normally excited by the widest apertures possible (there's probably a better way of phrasing that!), but for some reason the 35mm f/2 IS seems more interesting than the 35L. Not that I've used either, it must be said.
+1 with you for the 180mm macro. But what about that age old 50mm macro? that lens has been in Canon lineup since 1987. Canon is lagging behind Nikon in terms of lens releases as well now days.
Ah, that's a good point, but I have the 100L macro and the MP-E, so the 50mm macro is superfluous (and indeed I use the 85L + extension tubes for some stuff too).
I had both Mp-e 65 and 100L untill recently, I photograph snakes, butterflies and spiders a lot. 100mm L was too short for venomous snakes and bugs so I replaced that lens with Sigma 150mm OS recently. But for me in some situations 150mm is too long so I still need a 50mm/60mm macro in my bag.
Here are some photos that I took using 100mm L, 28mm and 50mm reversed with my old 450D.
Those are lovely shots. I would say (although I've not used it), the 180L macro lens would be better for all but the spider. I don't know how close reptiles let you get, but it's the best choice for larger subjects at a distance (butterflies are a classic example). You could try a 200mm lens with extension tubes as a cheaper alternative, maybe (but then you hit the maximum focus distance problem).
I've actually combined the 24-105L with extenders and extension tubes out of curiosity; it produces similar, if inferior results (large magnification without needing to get too close).
5D mark III, 50D, 300D, EOS-M; Samyang 14mm f/2.8, 24-105L, 35 f/2 IS, MP-E, 100L macro, 500L IS II, EF-M 18-55; 1.4xIII, 2x III + 2xII extenders; 600EX-RT; EF-M--EF adaptor.
Former lenses include: 70-200L f/4 non-IS, 85L II, 200L 2.8, 400L 5.6