Luzur
Good Sir
I totally agree with that.I'm not saying it's impossible to do; just that it would take a significant time investment that few people would appreciate in the end.
I've always wondered how much value can be gotten from borrowing code from a project like Dungeon Eye (http://www.dungeoneye.net/). Sure it's a completely different engine for a different game, but I can imagine that if the engine is a good, modern implementation with some very robust modularity one could plugin in a different combat system, and change up the UI a bit. Data formats will probably be crystal clear, and expandable as well. M&M never used the deepest systems ever.
The only notable Open-Source RPG projects/engine clones I know are GemRB (Infinity Engine), Dungeon Eye (Eye Of The Beholder), xBaK (Betrayal At Krondor), coab (Curse of the Azure Bonds), but that's about it. Strange no-one ever made a more modular blobber-engine, considering how they often had very similar gameplay.
There's also stuff like xoreos, Iris2, the many Ultima engine remakes (xu4, Nuvie, Exult, Pentagram) and OpenXcom (if you classify that as an RPG, which I do)
dont forget FRUA.