Formal proposal time, now that EGM is finished.
Anyone who has tried to make large, complicated rooms in Game Maker has been either bitterly disappointed or ingeniously crafty with regard to GM's rather limited room editor.
The quintessential example of a game that's fucking hard to make in GM is a Metroid platformer. There are two ways to go about the layout of a Metroid game in GM: There is the traditional method of creating a room for each little passage and cranny and using a complicated network of global variables to determine where you are coming from and where you are going, or there's the perhaps more advanced method of creating a large room of room-editor-lagging proportions and then using instances to denote regions to activate or deactivate as needed.
I have, in my longish and complicatedish game making history, made both a 2D and 3D Metroid Platformer, using a version (albeit a crude one) of the above strategies. It's quite possible that I've missed a strategy more form-fitting of Game Maker's... capabilities, but even in retrospect, I am not seeing a better way of pulling it off.
Enter EGM. Enter Overworld Resource.
Now that the EGM format is complete, we are capable of extending the existing room resource and adding a new resource on top of it.
Extending the room resource, we would allow for users to define their own "regions," as MrGriggs called them when he suggested them. These regions would be simple geometric shapes (possibly just box and circle) which, using a new set of room_region_* functions, the user would be able to test for being inside of. We then could have an instance_(de)activate_region function. Other modifications could be made to associate certain resource groups with regions, but this is part of another proposal I should probably post next.
Now, for the new resource, Overworld. Essentially, the overworld resource would allow users to place rooms into a larger room editor, the overworld editor. Ism would pre-render low-resolution previews of the room (she could fetch larger previews upon zoom) and allow users to place them in correct proportion on the Overworld map. A set of functions would be provided to get the full rectangle coordinates of a given room in the map, and contiguous rooms could be mapped together to allow quick searching for neighbors.
With that in mind, rooms could be easily reused without a complicated network of user-defined globals, such as room_to_left, room_to_right, room_above, room_below, or, worst of all, x_coming_from, y_coming_from.
Using the Overworld function set, a user could convert x-y room coordinates to x-y overworld coordinates by adding overworld_get_room_x(room_in_overworld) and overworld_get_room_y(room_in_overworld). The user could then translate those coordinates in the appropriate direction so as to leave the current room (essentially walking the character over), then make two calls to overworld_ functions to determine which room is actually going to be entered, and convert the overworld x-y pair back to room x-y pair after the room has been reassigned.
This topic is a good place for argument as to the exact functions to be made available, and this wiki page is a good place to put the results: http://enigma-dev.org/docs/Wiki/Overworld