Walls are a difficult subject in AOTR. They are now very expensive, twice as much as they were before. Nobody builds them anymore in multiplayer (way too expensive), and even in skirmish mode against the AI it is rather impossible to spend that much resources in a wall and not something else that will actively help us to win the game. In fact turtling in itself is possible, and even very efficient with a lot of factions, but not with walls because they're now bad for this, and for anything else to be honest.
I don't specifically remember the main reason for raising so much the price of those walls (it was a 6.0 feature if I remember well, maybe not). But now, I think it's safe to say that nobody is using them anymore anyway. The problem with reducing their price back to its normal state would be, I think, that annoying players would spam walls to kill the fun of the RTS game and basically do nothing until... the end of the world, by any chance.
I think, honestly, that if BFME had a way to win while turtling, walls at their actual price would actually be truly interesting. And that also means that you could add walls to factions that typically don't have them in order to make this kind of victory possible (just like Scandinavians in AOM that had shitty walls, but walls nonetheless). I would honestly give walls, in this purely fictional perfect world where the code works and everything is fine (completely rational I know), to all factions except MM. Tweaks on efficiency, hitpoints, upgrades, whatever you want, but make them useful, just like they were in ROTWK.
We could also think about other new win conditions, such as completing quests or whatever, but meh, more like a campaign thing. I don't know if this is even possible to implement something like that, or if it is "balancable" regarding other aspects of the actual RTS core of the game, but this is my suggestion. At least one new win condition in AOTR so turtling becomes a way to win, and so expensive walls still serve a purpose in the game (because they don't now, which is okay overall but a bit sad).