I'm very interested in this subject since I tend to think that e.g. a Brompton which has near zero trail would ride much better with some tweaking to the geometry. Having so little trail might have been an oversight which is now hard to rectify without massively complicating their production line, it might be an aesthetic decision to keep the pleasing lines of the bike. Or there may be other factors. For example, I imagine that slackening the head-tube angle would be a good solution, but at the same time I wonder whether the long stem design that folders tend to have and the weight of this part would then be more inclined to lean to either side... probably not... but really I'd have to test these adjustments to see how the ride quality was affected. But surely the brompton geometry is not yet optimized despite the company having existed for so long.
Having less rake on the fork is also an obvious option... perhaps it would reduce the natural suspension of the fork but more likely it would look a little less elegant aesthetically.