Now this would interest me greatly if it came to market.
When I went carless last year I wanted a folder for train journeys down to my Dad's - ride the 9 miles to Wolverton station, train to Euston, ride across to St Pancras, train to Ashford, ride to my Dad's sorted .
The regular Brompton is just too small for me and with my weight aboard flops and flaps like Larry Grayson's wrist in a semaphore class. It was more like riding a cross trainer than a bicycle. That gave me a serious downer because the fold is just to die for, and the finish is lovely, but most of all that wonderful fold.
On the recommendation of Road.cc I went for the Intercity Disc 9, the highest scoring folder they'd ever tested. Very fast, rock solid flex free platform, very nicely equipped and inexpensive, but...the fold. It folds more in the manner of a Tern, and while it isn't heartbreakingly bad (QR pedals help) its still a faff to carry, even for a longed armed orangutan like me.
So if Brompton brought out a 20", suitably scaled up and strengthened for larger riders I'd not hesitate to give them whatever they wanted for one. Irony is that as I ride the Intercity from Ashford International station out of town to my Dad's place the route takes me past the site of the new Brompton factory. Hopefully they'll look out the window, see me ride last and realise they're missing a nice chunk of market share and do something about it.