For me, a new 35mm f1.4 L is the most desireable here. It's easily one of my most used lenses. The current model does need a few updates. It's AF is pretty bad in low light. Change the Aperture blades for cirular ones, and an odd number please. It's not a weather sealed lens, although I've not had any issues in the rain with mine - hey I'm a brittish wedding photographer. It could do with the newer coatings, but other than that....leave it alone.
The next on my list is a new 135L, ideally I'd like an IS unit. Push the design to f1.8, while still keeping the stunning optical sharpness, contrast and colours. Shorten the MFD to 1 metre and weather seal. Add proper circular aperture blades (at f2.8 the Bokeh gets quite geometric) and lots of them for nice sunstars and it'll be another Canon Gem.
My third request is for a new 100-400L. I'd really like it to be a 75-400 so that it sits well with a 24-70L. Not fussed if it's push pull or extending...just get the tripod collar in the right place at 400mm. Make it a proper 400mm and not a 380mm. Give it a fast and accurate AF system, the current model is pretty pedestrian. A new IS unit is a given as are newer coatings. Stellar IQ and smooth bokeh are high on the list.
My 4th request is for a revamp of the 24-105L. It's a great lens, but it's old and pales next to the new 24-70IIL. At 24mm it's pretty awful, it could do with a new IS unit and newer coatings...will it be worth the price hike? Prolly not, but give it 5 years and it'll be a cheap lens again.
My last request is for a newer long macro lens. The 180L is a great lens, but it is a little soft wide open...but awsome stopped down. I'd like a 200mm f2.8 Macro. Fast AF and Hybrid IS. As sharp as the new 100mm L macro with the same smooth and sensitive AF ring style. Make it light, take extrenders well and pop the tripod ring in the right balance spot and Canon will have another gem.
One PPS request....a new 50mm f1.2L please....no point in labouring this...it's an ok lens, just not one of Canon's finest!