A rumor stated 7D may not have a direct successor. A FF/1.6 crop is not a direct successor, as it will not be compatible with the beloved 10-22mm and 17-55mm. Neither will an APS-H model. So, both could echo to that rumor.
My guess is more inclining towards APS-H. My reasoning is as follow: the extra reach provided by crop model is the major preference of the 7D camp. The 1.6 crop mode in a FF/1.6 crop model will be lower either in pixel resolution (if MP is less than 45) or lower IQ (if MP is higher), neither will be preferable to a potential 7DX buyer. However, a 1.3 crop model is a different story. Basically, you are giving up some extra reach for some better IQ (FF is an extreme case of this kind trade-off). In reality, 1D4 had been viewed as the upgrade of current 7D. If a 7DX comes with an APS-H sensor, Canon can pack more pixels (than current 7D) in it while at the same time providing bigger sensels (which implies better IQ) if MP is controlled below 27MP.
Besides, APS-H format provides an excellent test field for Canon. 7DX is viewed lower than 1D4, thus a minor failure in IQ can be tolerable which is good for experiment. A 21 MP in a 7DX (36MP in FF), if successful, will demonstrate Canon's ability in high MP models. A 24 MP APS-H will give a higher MP while still providing noticeable larger sensel size if other technology factors are same (to the current 7D sensor).
A 21-24MP APS-H 7DX would mean, 1D4's reach at about half price, more pixels for the heavy cropping, IQ in between 5DIII and 7D, possibly match 1D4's IQ if Canon could put any sensor improvement in it. Will it not be enticing to any current 7D user?
"Big splash"? In Photokina?