I do not really think that the combining of code is going to present as much of an issue as some of you are making it out to be. The engine is not going to be a clumping hodgepodge amalgamation of many other engines. The hardest part about the combinations is that compared to something like C++ you can't just add files to the project.
I do not know the full details now, I would have to figure it out as I go. There could probably be a "release" file which has what is considered stable for the time being. There would be a topic about Link and his abilities/reactions. This would get edited the most as Link has the most abilities in the whole game. As for adding monsters, there would be something that all monsters have in common - life, what to do if that life is over, handling collision events with Link's weapon, what to do if it collides with Link, etc. Once there is a default monster "class" all future monsters can derive from that class - this allows them to define all their own unique stuff but saves on coding.
To add a monster to the engine, one would modify the last stable release file so that it fit with the monster base object and so that they could test it with the Link object. Once they thought they had something usable, they would delete anything that was in their copy of the stable release and then submit it as a GMK file. Others could add it in to their projects and test it out and possibly work on it.
If that is too complicated people can just submit their GMK files which would not involve getting rid of all that was not in the base. If others like it and they can get it compliant, it can be coded into the standard release.
But really, you guys are missing the point. A community project is something that anyone can help out on and you are rushing to put it in C++ or something else which immediately limits it. MG-Zero, Minalien isn't here and Mamoruanime isn't going to be here as much. Infini also works on other things. You don't have enough support to handle it in C++ even if you wanted to.
Game Maker works, most people know it, there exists a wealth of knowledge on the forum so it is not like there is something that is going to completely stump anyone, etc. Of course it is too much work for one person, but there isn't going to be one person - it's open source. It is the best choice.