I have both the sigma 35 and 50 and can only say great things about them, so I would love a wide zoom to replace my 16-35/2.8 as well. (Plus a 70-200 sport to match Canon's MK2 hehehe)
I presume you were referring to an ultra wide? They've already produced a super fast wide angle zoom in the 24-35
And yeah, upgrades to the 12-24, 24-70 and 70-200 would be fantastic to see happen, even if they chose slightly different focal ranges and pulled off some more interesting firsts.