Ok, so now that we have a clearer screenshot of Isshu, I believe more than ever that this is only a portion of it.
If you look at both icy mountains, you will notice something very cool about their positioning. First, they are exactly opposite of each other. I believe that these mountains will hold either legendary within it, since it would make sense that opposing mountains represent Yin and Yang.
Then, you will also see that "island" located in that ring of water between the two rivers. This is placed to be perfectly in the middle of both mountains. It also, as others have said as well, looks like a shrine of some sort. Not to mention the bridge that connects the two mountains as well as cuts through this shrine.
As Mt. Coronet was introduced fairly early into the game for DPPt, one can assume that these mountains are meant to be introduced early on as well and with the shrine. The player would either navigate around them for the time being, or perhaps explore one or the other a little as done in DPPt, whereupon they would continue on their journey. There is obviously a river of some sort along with other bodies of water on top and leading down the mountain on the right. But they are far too close to any of the most likely starting points for our character for us to be able to surf them yet when we first reach them.
Thus, I believe that this shows that there must be much more to this region, perhaps even more than Sinnoh was compared to our first screenshot of it.
The two rivers also support my point, for there is no marked beginning of them, so having only what we see in the pic be Isshu would leave unfinished work on GF's part and unanswered questions on our own.
Also observe how both rivers at the top of the pic are beding down from the Northwest. And what are the two primary directions which may most likely yield the rest of Isshu? The North and West.
Now allow me to address the new land distinction which has been revealed in the clearer pics: that raised land with all of the holes which seem to be the entrances to caves. Perhaps they are ruins of some sort, but no matter what, it is distincly levled out at the top. To my knowledge, there has never been a plateau used as a geographical feature in the franchise. So, my belief is that this is some sort of plateau which yielded to the discovery of artifacts of some sort within it, possibly relating to the two aforementioned mountains. What the pic cuts off is most likely a northern exit off of the plateau where the region continues on as usual, with those two rivers either merging into one as we follow it north or with one possibly end before the other.
Being that next month's corocoro will involved some new info that will include information on the game's story, I hope that it will shed some new light on the situation here in Isshu. If we're lucky, we might even get a full pic of the region then (even better, with some more new pokemon
).
Until then, I will daydream of my theory.