I agree with atleast making the prototype in GM/some other engine. We need to see how the idea works out, and if our dedication is strong enough to actually realise a project like this. (Even a simple game takes a lot of work to get it polished.)

I've made some games in the past, in GML and in other languages. Unfortunatly I'm very busy with the combination of College and my own projects (AndroidGIP, Serversoftware, etc).
However, in a few months my workload will probably cool down a bit, and if this project is more than just a plan at that time I might be able to contribute some code

As for the gametype/story, I think depending on the story and engine we decide to go with we'd better keep it 2d. (3d in GM is hardly supported, with only a few basic functions). Something like a platformer/rpg.

Another thing to consider, is thinking up something that sets this game apart from others, even if the quality is lower than some commercial games. Some gameplay mechanic that wows the users, like the portals in Portal or the gravity in Super Mario Galaxy.

Well, in whatever way this might turn out, Happy coding/drawing/composing/etc. everyone!

P.S: I wish I had a cake generator in my house, cake tastes a lot better than the food in this place XD