"And there’s an understanding that OpenMW’s “intention” isn’t solely to be a way to play Morrowind, but also to implement a generic RPG engine and editor that uses Morrowind’s gameplay and file structure. For someone to make their own game with, not using any of Morrowind’s assets. (I’m not sure if any such game exists, though)"

To be honest, every project of this kind claims that. And in reality, everyone knows 99% of usecases for such engine is to play the original game. Sometimes there are new IPs being created on such engine. For example recently there's been some "boomer shooters" coming out using engines which were made originally to play classic DOOM or Duke Nukem 3D. But such cases are rare.