I would love a 12mm f/2.8 prime lens, except I have the feeling that lenses, especially wide-angle lenses, for the current EF mount will go obsolete more quickly than usual in favor of lenses for the RF mount.
The RF mount, with its 20mm flange focal distance, versus 44mm for the EF mount, enables optical designs which are of equal or better optical quality while using less super-expensive optical glass. Less glass means that the lenses are not only smaller and easier to use, but also means that the lenses can allow customers to save money and/or Canon to make a bit more profit and thereby invest that added profit in developing new technology faster and better for all of us to benefit.
The RF mount is the future, a very, very bright and promising future. I am especially happy Canon chose to go with 20mm to maximize the strength of the mount while still achieving an optically-optimized mount solution compared to which the previous EF mount was at a 220% disadvantage (44/20).
This is a huge deal, as attested to in spades by the incredible optical quality of new RF lenses like the f/1.2 50mm and f/2 zoom. Those lenses would have been unmanageably huge and nightmares of optical complexity just to achieve equivalent image quality when engineered with a 44mm flange focal distance in mind.