Here's my short list... lol.
* steering rack & pinion conversion kit.
Maaaybe
* fuel line cooler like OE part.
Why? It really isn't worth it for what it does and would cost a lot.
* hard mounts for alternator (stop the twisting).
These already exist, but if they are hard to get or too expensive, I can come up with something.
* radiator shroud that doesn't warp.
Got that covered with our aluminum radiator design. Will also have integrated electric fan option.
* a proper steering box rebuild kit.
Dunno about that one.
* custom fuel rail (S38)
with FPR fitted
This can be arranged.
* properly engineered reinforced pedal box.
Hmm...
* balanced exhaust cam sensor rotor with opposing steel and brass inserts.
What tooth configuration?
* M badges that wont fade.
I can have these made in cloisonne if people want to pay the money for them.
* M5 exhaust tips.
Probably not.
* carbon fiber front air dam splitter.
Try vorsteiner
* aluminum rear trailing arms.
That would be cool but cost a lot of money
* finned big diff cover (like the Z3 one).
Again, money and lots of it.
* block off plate for G280 reference sensor holes. For those who've converted to stand alone EFI.
I think you may be the only one who would buy that.
* adjustable sway bar links front/rear to fit Dinan, RD, GC, etc.
I'll look into it.