Bromptonesque frames are available aftermarket fabricated in stronger, lighter materials than the HiTen steel Brompton uses; here in titanium:
"The best defense against copying is innovation.” Will Butler-Adams
I talked to a guy up in Portland that believed that simply swapping steel for titanium induced too much flex, and a proper frame build required oversized tubing. He had a prototype built which was nice, but I think he only did it as a one-off. Perhaps he'll build more someday.
I've got the contact info somewhere.