There are three problems with the existing forks:
1. They use a 74mm hub. This is a standard with Brompton but proprietary in the larger scope that has accepted 100mm as the standard. Mini-velos, for instance (other than those produced by Dahon), use the same 100mm standard that all other bicycles use.
2. They use cheap materials that are sub-optimal for the purpose - aluminum or hi-tensile, instead of chromoly. The frame is chromoly so, why would the owner want a fork made of a rough-riding, non-dependable material such as aluminum or a heavier, weaker version of steel such as hi-tensile?
3. Lack of dedicated braze-ons/eyelets for racks. If you want to use a rack on the fork, you have to share the mounting bolts with the fender.
The widely-copied frame is the Dahon Speed (sometimes referred to as their K-series). This frame is copied by Fnhon as their Storm model (also known as the KC2018), by Crius as their Master-V (the 'V' for V-brake), by Banian as their Wind Speed model and by numerous other companies. This frame is light, strong and uses the 406 size of tire (BMX size, which is very common and comes in a multitude of offerings).
There are folding bicycles which don't suffer these problems. KHS's F20-JJ model is one, but which suffers a problem of handlebars that do not adjust for the rider's height.