Your plans for Riverrun 2.0 between the Red and Blue Forks got me wondering, just who owns that land right now (or before Wo5K)? And why haven’t they tried to build a castle there already? Hell, if I were Lord of Harrenhal, I’d start angling to move north right away, closer to the Trident. Even a ringfort between two rivers would be better defensively than Harren’s folly.

We don’t know, exactly. Oldstones, for example, seems to have remained in ruins ever since the fall of House Mudd, which is kind of odd given that viable land is something of a premium. We also don’t know who the lords of Fairmarket were/are after the Hoares were chucked out. It does seem like a lot of the land between the Red Fork and the Blue Fork is underutilized. 

As for Harrenhal, I think it’s a bit of a white elephant thing. If you’re the lord of Harrenhal, so much of your income is tied up with maintenance that you can’t really afford to buy real estate.