First of all I'm surprised how much lore there is on this
Regarding concrete planning:
- No beacons, as reasoned by @Kjolsen.
- Wall: I support @Eriol_Eandur's view on this. The river basically "is" the wall or just as good as defensive structure. So I'd say no wall.
- As @Ardelenia pointed out, a bridge over the Limlight seems to be canon, so add that and follow @Eriol_Eandur's suggestion to move the northern of the keeps further up, north of the Limlight, where it enters the Anduin
- Area between Cair Andros and Rauros: The point that no real fortification is required seems reasonable. First off, I think the Emyn Muil are pretty much supposed to go as far as to touch the Dead Marshes and Nindalf. As we know very well from books and movies both regions are very hard to navigate, especially for a dangerous amount of troops.
- Age: Gondor has neglected fortifications and observing fiends for a long while, so it is rather likely that fortifications north of Emyn Muil were abandoned rather soon, as in the lands immidiately behind the fortifications there was no populated or otherwise very valuable region and thus may well have been deemed not worthy any extensive protection. The quote from @Ardelenia that the Eorlingas might have occupied the fortifications is a good point. But I doubt any of the forts were occupied during the War of the Ring as noone seems to have noticed neither the fellowship travel down the Anduin nor the orcs/uruks in that area (besides Gwaihir and Gollum). Now our server is set 17 years before that though and it might be possible that the forts were occupied then. Rohan might have withdrawn the troops as the dangers were increasing and they rather wanted to protect the densely populated areas instead of that crossing (resp. just answer to the threat of Mordor rather the one of the Balchoth).
So, old fortresses, ruined by time and weather, maybe with rohirric wooden fixes in the North Undeep.
Regarding terrain:
Between Emyn Muil and Lothlórien it is WP terrain mostly which will eventually most likely be revamped. Among the Emyn Muil we have Sarn Gebir which we're still missing about. But that requires some height difference to work with, the most likely scenario is that we at some point lower the Falls of Rauros for an insignificant 10-ish blocks (it will still look tall) and instead use those up inside or near Emyn Muil. So for that, and also as there's a few other flaws about the Emyn Muil, they will also receive a major facelift eventually, as will Ethir Onodló and the Dead Marshes (along with Mordor). But the question is when we might get to that. Currently we're finishing Gondor, and focus is on Moria and will also remain for a long time. By then Mordor will be ready, and even after that other things will be a lot more pressing e.g. a #ShireRevamp. So, especially if it might be 'only' punctual fortresses and no wall it might be more reasonable adding those rather sooner than later, as fortresses can easily be taken out of the terrain when it's about revamping it.
I don't see how this project conflicts with Mordor work tho, it does not occupy any map tile that is part of Mordor.
WorldMachine is most likely not really a good solution for this, a major aspect of it is water which is still rather new to WM and is most likely not compatible with WorldPainter and/or can be controlled well enough to get what you want. Also the shape of the area is a narrow, unshaped band through the terrain; you'd have to rework quite a bit of additional terrain around the Anduin as you can only import entire map tiles (if you look at dynmap, that is the fine white dashed lines you see). Furthermore there's no easy way to hide the border between WP and WM terrain which would be visible very well; with Mordor and Tolfalas it works as the borders of Mordor are nicely orthogonal, and Tolfalas is surrounded by sea.
Then another problem is that it seems like we have rather too few people around currently to make decent enough progress on all our projects. And yet another project that will withdraw a couple of Artists from the main projects won't be of any help...
So, I'd really like to give you a go on this, but right now it is fairly difficult...