If I wanted one I'd get it now.
If a MkII comes out then it will be much more than you can get the MkI for so you have a different set of factors contributing to your purchasing decision.
Would you buy a MkII now for $2,200? Because even if they were available they wouldn't be selling for less than list, and wouldn't be for some time after release. Look after your MkI, keep the box and bag, receipt, warranty and paperwork and even if you want to upgrade in the future you won't lose that much.
Besides, lenses are for taking photos and imagine the images you will miss in the mean time.
If I wasn't in a rush I'd see what the rebates brought, but if you are not in a rush do you need it...............
Normally I'd agree but I wouldn't be shocked if the new 35mm 1.4 isn't sold at that big a premium due to the Sigma.
I think Canon have demonstrated on pretty much every occasion that they don't care about Sigma. Sigma are not pushing them to do anything, if they were we would have seen a 50 f1.4 replacement a long time ago.
I believe the bigger picture is Canon think the relevance of fast primes has waned and they don't carry the "system" clout they used to. Killer specialty lenses like the 24 TS-E and 17TS-E, and zooms with the IQ of the 70-200 IS f2.8 MkII and 24-70 f2.8 MkII are not only expensive but they are good sellers with much broader appeal. I well understand the fast prime "look" that can't be replicated with f2.8 zooms, but it seems to me Canon don't really care too much and have moved on faster than some of us, lets be honest the most compelling reason for fast wide primes was not dof control (though I am not denying it's importance sometimes) but it was to compensate for awful film iso speeds and that has very much been put to rest with even current camera iso capabilities, I suspect Canon consider medium speed primes with IS that are small have much more appeal and earning potential, the new lenses are often video orientated and that is what Canon thinks is more important to them.
I don't really feel like I need IS on a 35mm lens for what I shoot. I'd rather have the wider aperture for the 35mm focal length. Handholding at 1/30th works ok with a wide-ish lens like a 35mm, and that is plenty slow for me. I don't have much need for 4 stops of IS to allow me to shoot sluggish 1/2 second exposures handheld because most of my subjects are alive and moving way too much for that, even if trying to hold still. When I do occasionally shoot still life I am fine using a tripod or resting the camera on something.
What I do like about a brighter 35mm is that instead of having to jack up the ISO to the point where I notice noise or lack of detail in crops, I have room to open it up and gain a stop of shutter speed at equivalent ISO compared to what an f/2 lens could do. And even wide open, the DOF on a 35 usually stays wide enough to work with, and does not get so shallow that you are stuck with that look where practically nothing in the image other than a couple of eyelashes, seven pores and one pupil is in focus.