I'd be more inclined to attack this from the opposite end in identifying what on the bike doesn't perform as you'd like and researching alternatives...
Brilliant Bikes are good but tend to stock fairly mainstream / Brompton supplied / "approved" bits,
Kinetics offer some really progressive and well-executed upgrades / conversions, but aren't cheap.
There's plenty of Chinese tat on the usual marketplace outlets, but much of it is of questionable worth.
Personally on
mine the only off-the-shelf mode I've done are easy wheels as the originals are crap. It's also had custom brass plates to protect the areas of the frame that are prone to wear / damage from the fold and are pitifully unprotected from the factory, an extended front mudflap and a modified phone mount for my choice of GPS unit..
I'd love to improve all the other design flaws that cause the little tosser to continually try to eat itself (seatpost tube sleeve, piss-poor hinge design) however sadly these appear outside my abilities to resolve satisfactorily. It could also do with the frame cavity waxing to arrest the obvious corrosion that's developed despite Brompton's claims of factory-applied rust proofing.
All pretty basic and uninteresting, but arguably of far greater worth than festooning it with cheap trinkets just for the sake of it.
