Having actually researched lens design here are some realistic dream lenses that would be top notch quality wise, supertelephoto level quality:
EF f/2.0 Zooms
19-35mm f/2.0L
This lens would be an ultra fast ultra wide angle lens, while it couldn't go to the widest super wide levels, it would serve most people's wide angle needs while providing excellent aperture and excellent quality. Note the lack of IS which would be too difficult to design in.
95mm filter thread.
35-70mm f/2.0L IS
This lens would be a normal zoom, without the added wide angle we're accustomed to. The most difficult part of designing a normal zoom is the wide angle, so by getting rid of that you can essentially create substantially more extreme lens designs.
85mm filter thread.
70-150mm f/2.0L IS 1.4x
This lens would be a larger and faster version of the 70-200mm f/2.8L IS II, leveraging the built in 1.4x telecovnerter system to make up for the added reach.
82mm filter thread.
EF f/2.8 Zooms
35-105mm f/2.8L IS
Again by eliminating the wide angle we can get more extreme lens designs, as I mentioned this would be supertelephoto quality. This would be an excellent portrait lens.
85mm filter thread.
EF Primes
35mm f/1.6L IS
Make it much sharper than the 35mm f/1.4, lose a third of a stop, add IS. This lens would be large.
77mm filter thread.
45mm & 60mm f/1.4L
The design of a 50mm fast prime is very challenging, and nobody has ever made a 50mm that has supertelephoto level resolution wide open, or even come close. You must move up or down the focal range to get better results, so these would be the answer.
72mm filter thread.
135mm f/1.8L IS
This is an existing Zeiss design, slap IS on it and tweak it to reduce purple fringing and it would be a mini 200mm f/2.0 IS L, make it a miniature supertelephoto in appearance with full supertelephoto quality.
77mm filter thread.