Individual bricks have the notion of points of rotation: centers around which the brick rotates on a given plane. This lets, for example, the various hinges move in a "natural" way. It would be nice to have the ability to set similar centers for submodels, so that they also move in a "natural" way.
At the very least, it would be useful to specify a single brick in the submodel and say "use this brick's centers when rotating the whole submodel". For example, say you had two submodels that represents segments of an arm. Say one segment contains a 481 piece, the other a 482 piece, with the idea that the two segments connect at this hinge. If you could specify that one submodel rotates around the 481 and other other around the 482, it would be much easier to get the arm-like action right in the main model editor.