I've been thinking about the triggers that people will need and it raised some interesting points about collision detection. Mainly, we don't have to collision check every entity in the game. Instead, I'm thinking about providing collision type lists - so that the user creates an entity and registers that it can collide only with a certain type of entity (or entities). This would allow you to create your player and say "I only want to collide with entity types for the bad guys ('boss', 'grunt', 'etc')" and the game will only check tese collisions. The different bad dudes won't try and collide with each other unless you specify it. Collisions could come in the form of a trigger or just a built-in test which raises an event. I'll probably go for the later version of this as it makes more sense.
I'll be adding in TileMaps soon. Being a 2D game engine tilemapping and the scrolling functions are paramount (IMHO). Tilemapping will be an interesting beast - I need to allow it to be intelligent enough for a user to pass an array-like structre to use for the tile offsets. The closest thing GM has is the table, so I might create my own array.