It's truly an amazingly crowded field, though IMO, because all of the modern 35 1.4 lenses perform well enough for most photographers, I chose my Sigma 35/1.4 because it was inexpensive compared to Canon's (and now Tamron's) offerings.
In celebration of Tamron's 40th anniversary of the SP lens series, the Canon EF-mount SP 35mm f/1.4 Di USD has been created to showcase their lens-making capabilities. This flexible wide-angle prime is ideal for general shooting applications, ranging from street photography to event shooting...
If they can make a lens this good already for SLRs, how much better it could be without the restrictions of the longer flange distance.
As far as where Canon and Sigma is heading, it seems though that for a mirrorless mount, they would rather just make it even faster.
Which means doing a 35mm f/1.2 and raising the price further, rather than reducing the size and weight (making it more proportional to the camera bodies) and keeping it at f/1.4.