Thanks everybody for your useful answers.
My current lens (24-105L) weights 640g, which is way to much. So I won't consider the Tamron SP AF Di 28-75 f/2.8.
The interesting options are :
- Canon 40 f/2.8 STM: maybe not wide enough, but very compact, very lightweight (130g), and quite cheap. But I have seen reviews that criticized the autofocus and the overall image quality.
- Canon 35 f/2 IS USM : which could be a good FL for landscape with a great aperture. (335g)
- Canon 28 f/2.8 IS USM : good FL and quite lightweight, price quite low (260g)
- Canon 24 f/2.8 IS USM : maybe a bit wide, but weight is good (280g)
I've thrown a look at the voigtlander... What FL could be interesting for landscapes with the 6D ? 20 is maybe a little wide ?
Between the Canon primes, what is the best lens in term of image quality ?