One of the things I want most out of a mirrorless full size EOS is the ability to make 24mm Pancake lenses that sit as much behind the flange as in front.
There isn't much space back there as is, it wouldn't be hard to keep the same EF mount and still have mirrorless lenses incompatible with mirrored bodies, the only thing you have to keep the same is the contacts, which are mostly out of the way sitting on the bottom, which wasn't a problem for EF-S either.
The real question is whether a 24mm pancake is really worth all the trouble, but if you're making a camera with no mirror anyway...
(and yes that an "EF" Full Frame 24mm Pancake that I'm talking about, not "EF-S", which is already coming)
Not needed. Exists already ... EF-M 22/2.0. Small pancake. Great lens. Excellent value.