Please make it an L lens with IS
If they're doing it, I'm pretty sure it won't be L (otherwise they're going to totally kill the f1.2), and I seriously doubt (but I'll be happy if I'm proven wrong) it will be with IS, I don't recall any bright 50ish lens from any first or third party manufacturer (Tamron 45 f1.8 VC is not "bright" in the scope of this discussion) to ever having IS. Actually I think that Tamron (for my knowledge; correct me if I'm wrong) is the only 50ish FF (or even not FF) lens of any brightness I can recall having it.
If they do it (and I hope they will!!!) price needs to be at max half (or less) of the f1.2 so 1000/1200€$£ish; an used 50 Art can be found for 400€, for a brand new 40 Art (which is better then the RF 1.2) I just paid 755€, and (if there will be third party RF lenses, which I also especially hope) a brand new 50 Art DN is 900/1000€ new, so if the Canon would cost more then 1300/1400€ (and again, if third party RF will be available soon) it would be much less attractive (given that the optical performance is on par with the Sigma's, which is of course yet to be seen).
Personally, given "1" the price of a comparable third party lens, I'm available to pay "1,2" for a Canon lens, to get proven and everlasting compatibility with all future bodies even in the next 15/20yrs, surely perfect AF algorithms, in camera corrections baked in for video (for RAW's of course no need of in camera correction, as long as I can find them in LR/C1), a supposedly better service and availability of spares, etc; if the price gap, at the same quality, is superior, I'll personally always go with a (proven, like Sigma) third party alternative.