Don't forget that Pentax have a 21mm pancake. It's made for the K-mount, so 45mm flange distance, but it only covers APS-C, so maybe that's how they got a wide-angle pancake to look so good (it's not as nice as the EF 40, but it's as good a wide-angle as you could expect in a pancake).
Seeing as we've already got 40mm, then 50mm and 35mm are probably out. 80mm makes a nice 'pancake' for 6x6 (well, my Biometar (Planar) is the size of a nifty fifty, but for MF that's pretty small). I doubt we'll see one for EF.
So 28mm is the next logical point, but it'll have to be f/3.5 (probably) to get it small, and besides, we've just got a new 28mm f/2.8 IS, if they make an f/2.8 nonIS pancake it'll kill a few sales of the IS. Ditto that for 24mm.
Unless of course it's an EF-S at 28-35mm, then FF users will still have to go for the IS?
And whether Canon can make a 21mm and still keep it 'pancake' and cover FF at the same time? Probably, but I don't think they'd try.
So my guesses: 90% probability the Shorty Forty is all we get. 7% for an EF-S 28mm. 1% chance each for a 21 or 24mm, and the last 1% is Canon surprise me.