Stepping back a bit. My best guess is that this Plains of Greyflood region will be shaped like a sort of right triangle. The southern border of the triangle will run along the cliffside below the Gloomglens, then head west from Maur Tulhau and Cliffsburg, following the road leading west from the Fishing Hole. The east border of the triangle will run alongside Enedwaith and Eregion. The west border will be the Hoarwell and Gwathlo Rivers. Altogether, I expect the zone to be about comparable in size to Enedwaith. Again, note that this is speculation; I think it makes the most sense, though, given the what I saw, the landscape that's accessible on live, and the geography of Tolkien's Middle Earth.
I imagine the zone consisting of four areas. There will be some hills, gullies, and ravines in the south of the zone, perhaps with another hobbit village or two. North of the hobbity hills will be the Swanfleet, a marshy area which is canonically similar to the Gladden Fields, where the Stoors used to dwell. I imagine the north end of the zone will consist of the area between Eregion and the Angle, and will probably serve as a transition between the two; I imagine there might be some elven ruins in this area. And the western end of the zone will probably be populated by a ruins of Arnor and Cardolan; there used to be a line of forts along the river, the ruins of which we'll probably be able to explore.
And in the southwest corner of the region, I expect we'll come to the ruined city of Tharbad. Tharbad used to be a major garrison city until it was abandoned about a century before LOTRO takes place. Now the city lies in ruins. I imagine SSG will turn the ruins of Tharbad into a major enemy based, perhaps with a couple of instances or a raid attached. Seems like it would be a perfect opportunity for that.
I envision paths leading into the Plains of Greyflood from the King's Way Gate, from the Fishing Hole, and perhaps from Maur Talhau. I'm sure there will be a path or two from Eregion as well, perhaps around Mirobel. There will probably be a path from Tornhad in the Angle. And I suspect we'll eventually be able to cross the ruined bridge Tharbad into our NEXT new region.
I suspect there could be a lot going on in this region. There might be some elves in the north, part of the company that followed the Fellowship from Rivendell. There might be rangers, either having come from Tornhad or on their way back to Tornhad from Gondor, depending on whether this is a lower-level zone or a high-level zone. There will most definitely be hobbits. There may even be Rohirrim, since the Rohirrim will ultimately come to inhabit this region. There should be plenty of elven, human, and hobbit ruins for us to explore. As I said, I suspect there will be a large enemy presence that's gathered in Tharbad since Boromir crossed the city's ruined bridge. Depending on when in the game's timeline we come to this zone, the ringwraiths may even be in Tharbad, having been swept downriver from the Fords of Bruinen. This area has deep layers of history to explore, too, as well as plenty of opportunities for undead gooblies. This is a region that's seen a lot of tragedy and death, after all. And of course, I imagine we could see bandits, bugan, Dunlendings (friendly or hostile), orcs, goblins, possibly Angmarim and Black Numenoreans and Dourhands... the possibilities for this area are truly endless.
I have one more image for you. It's an altered version of Valather89's map from earlier in this thread; hopefully they don't mind me piggybacking off of it. I've marked out what I imagine to be the borders of the Plains of Greyflood, as well as what I expect to be the next two major zones, which I'm calling the Cardolan Marches and the South Downs. I think it makes sense for us, now that we've started to follow the North-South Road northward, to connect it all the way up. It certainly fits with SSG's current focus on expanding Eriador and filling in Tolkien's map and their desire to explore sunnier and less dire regions. I could see the epic picking up again as well, taking us back to Eriador to discover its remaining dangers, parallelling the hobbits' return to the Shire.
To lend some evidence to this idea, I'll point out the fact that the terrain in the Gloomglens wasn't the ONLY terrain SSG had to correct in today's patch; they also had to fix the terrain on teh south side of the Bree-Land Homesteads. If you go to Andrath, climb to the top of the ruins, and look south through the gate, you can see ruins that are floating in midair. They've definitely been doing some work south of the Bree-Lands.
I think we're probably going to finally get our River Hobbits alongside one of the upcoming regions. River Hobbits... as in the hobbits that still live in the area of the Gwathlo, Glanduin, and Hoarwell Rivers.
And my guess for the next area after the South Downs? My money's on the Southfarthing.