I was vaguely remembering a few projects back in the day to build some libraries that you could simply use for various aspects of the game engine (battle system, movement engine, etc.), and I was wondering if any of those ever came to fruition? In particular I'm going to have to program a Secret of Mana/Legend of Zelda-esque battle system, and it seems like something that would be common enough that other people would have implemented it a hundred times over. It seems like there would exist a well optimized implementable framework that would be available, but there isn't yet. While I understand the importance of knowing how all your code works, especially the most "basic" systems, it would still be useful to have a well optimized framework to compare against and learn from.

As far as I know, there have been several, but they end up being used solely by the person who made them in the first place. Like Lithonite, RadLib and that FFVI engine.

There's a few of them here, and some of the tech demos use a few of engines like those too.