See, there's the flaw in your logic. All those "little" changes require new tooling. If you want a trap door, they have to change the tool for the original base plate (to accomodate the new door and hinge) as well as a new tool for the door. Ripping out the interior to replace it with new/different chairs ... same problem.
The only way for it to be done cheaply (I hate that word, makes me think "low quality", let's say cost effectively
) would be as a straight up repaint/redeco.
I would much rather see that tooling money go into a new design that was specific to the line rather than trying to re-appropriate something from another line to wedge in.
In regards to your propsed Charlton pack ... I would buy that in a heartbeat!