So my question, is since there basically is no alternative to Game Maker, is how do you plan to handle maintaining one up to date Game Maker source file?
Consider that you'll have multiple people working on different features in their own style. How would you keep track of who's doing what? How would you combine the various components into one file effectively?
You guys understand what you want to do but how do you plan to actually go about it and keep it organized, efficient, and functional?
Before you decide on what the game should be about, first figure out how you're going to make this engine in gory detail. It doesn't particularly matter what the game is about, so long as the engine base is both good and stable then you should be able to expand upon it with the needed features for a specific game.
Keep in mind I'm not saying write the whole thing then decide the game. I'm saying know exactly how you will plan to write it as a community and then decide the game.