FF in a mirrorless - I don't think so. Why? Because the idea of mirrorless is to miniaturize everything right? So if canon releases a mirrorless, it likely would be a compact body. FF requires larger lenses because the lens needs to project a larger image circle. So FF mirrorless would be small body and large lens. The ergonomics on this would be a nighmare. I too like the idea of having a compact FF, but having a beer can attached to a deck of cards probably won't work out well.
FF in a Rebel - probably not with this next round of rebels, but eventually yes.
FF in sub 5D mark iii - if the "entry level" ff rumor is true, this will be the case. My guess is that it will be an xxD camera, not an xxxD camera.
If you look at what Leica has done with their manual lenses for the M9 and wrap a AF USM motor around them, are they likely to be as big as EF lenses of equivalent maximum aperture ?
One of the reasons why EF glass has to be that big is because of the lens backplane to focal plane of sensor. The further it is, the bigger it has to be. For a mirrorless system, this distance can in theory be reduced to almost zero. Practically, about 1mm. Current EF lenses and EF-S lenses is in the range of 20-35mm. M4/3 lenses about 12-20mm. Leica somewhere in there. Flange distance does not directly co-relate to the backplane distance, though it will have some influence in the overall scheme of things.
But it is not very likely for now that the Canon mirrorless will be FF. Too big a jump for the market to accept and also, the limited range of lenses that's specifically designed for such a 'high-end' mirrorless camera to take-off successfully. It will possibly carry the same sensor as the G1x if market penetration is the objective. However, if Canon decides to do something that turns the market on its head, and grab some of the Leica 'halo', a FF mirrorless system is not out of the question either, they have the sensors as well as the resources already. Just need to create additional adaptor mounts that allow the use of EF and EF-s lenses with full AF functionality on such a camera.
As for a sub-5dMkIII, that's very unlikely if the market differentiation is to be achieved. For Canon, 'entry' means the lowest spec's that the market is willing to accept. And that was the same thing that prompted the creation of the original Rebel. It had the 'minimum specifications' which the market was willing to accept for the targeted market prices of sub US$1000/- back then. For this case, as it is now, the entry specifications is more or less what the current or upcoming Rebel carries if the target market price is to be US$1500-US$2000 ( which I assume is the market threshold for a 'basic' FF DSLR here). The 1100D is below 'entry' for most people, which it shows in comparison when you look at the numbers moved/sold compared to the current and past Rebel models.