I like the idea of constraining yourself to one of a number of things, especially for the twist reveal. But I think you need to be careful when you have such tight constraints - I almost gave up on the game without realising that the dark grey square was a movable object (it just looked like scenery and the only other grey square in the game is something that you can walk over). You could try putting it just behind the gate, so players will have to push past it to get in to the new area?