Hi,
I'm not going to discourage anyone from opening up a new parts supply for our cars but if none of the mainstream suppliers are planning another production run then maybe the market isn't as big as we imagine ?
If you think of the UK, there's Lotus Supplies, SJS, Paul Matty, Kelvedon as big players plus the guys on your side of the pond who could band together for a production run and share costs. The moulding will presumably be in place now so it's a casting program and you'd leave machining costs until someone actually wants to buy one, just keep the rough castings in stock. If they aren't interested then maybe it's because they expect to have them on the shelf for years and prefer faster moving stock ?
I'm not convinced they are a service/wear item, mine are the originals from '72. The bearing surfaces are good and the only issues have been one radius arm bolt needing to be helicoiled. That turned out so well that I did the rest and hopefully that's sorted for my ownership. I'd guess that my car isn't unique and the only time folks replace them is if there's been lack of maintenance, the car driven with worn bushes, bearings breaking up, loose lower link bolts, etc.
It's going to be tough to come up with something lighter or better designed for the OEM suspension, but you could consider a carrier with facilities for a twin link suspension for anyone fitting different transmissions. LIkewise somewhere to bolt on a rear caliper, maybe with an adapter to allow different caliper makes/bolt spacings/disc diameters ?
I think you'd have a larger market with something like that rather than a straight replica of the OEM carrier. It should be usable with the OEM suspension/brakes but also for those wanting discs, etc.
Stronger stub axles and internal spacers are an entirely different case, I reckon you'd have no problem with that market. I think you'd need to sell as off road/race components, I don't know how product liability would work if you sold as road use parts.
Brian