I'm surprised more front-wheel-drive vans are not stretched like a limousine to add room. I've seen (and posted) pictures of FWD Transit and Ducato vans that were stretch about 4 to 5 feet by adding more or less the length of the side doors.
The Ducato Maxi van that was stretched to make a camper was awesome -- ended up around 25 to 26 feet long yet looked fairly normal from a distance.
I've always thought conventional car-based limos are stupid. All that effort to increase the size inside and you still have to enter and exit from a tiny rear door. Vans are way better place to start for limos.
A limitation with stretching some modern vans is probably that the body may taper too much from front to back to look right. Ford Econolines are a good example in that the rear windows (or side panels) get smaller as you move towards rear. I photo shopped my Econoline years ago and it was ok from afar but dimensions didn't match perfectly. Stretching would be difficult and costly.
Vans like the Ducato, Boxer, ProMaster etc... seem to be much more uniform front to back. That and FWD should make it much easier to work with.
A stretched Maxi like this one would make a great camper for us; just smaller than small Class As but much tighter overall.