I've heard such rumours ever since the 17mm came out - unfortunately all from sources I'd never heard of before and firmly (IMHO) in the wishful thinking category (i.e. not even CR1 here).
The 17 is extremely wide when you look at the full image circle, so there issues in the sheer amount of rectilinear distortion if you made it wider. i.e. even if perfectly rectilinear, it would look massively distorted at the edges. That said if Canon made a TS-E15, I'd want one...
In talking to Canon about the three new TS-E lenses, they effectively regard them as the 'third generation' of TS-E lens designs. Where they really shine is at wider apertures and with larger movements. Their tilt performance is noticeably better - I'd suggest that's much more relevant for the longer lenses.
I can see a Mk2 TS-E17 that's better over the whole image circle at say f/5.6 - maybe even with T/S data in EXIF (but I still think that unlikely).
In recently comparing the TS-E90 old vs. new the areas of improvement were obvious on my 5Ds - I suspect we won't see the 17 updated until there is at least a 120MP FF sensor in Canon's range (yes please - file size and FPS is of zero concern for my work ;-)