I think it would be quite simple really to cover up the upper arms in beast mode and have the shoulders for robot mode.
All they really need to do is have a swing-able cover that snaps over the inner arm in beast mode to make up the black gorilla arms, and when transformed, the inner cover can swing around up into the outer cover to make up the shoulders for robot mode. Can you see what I mean? Id be happy with that.
Now if only i could write to Takara with my idea. ha.