This is a great question. Unfortunately, I think it may not happen for a couple of years. Rivian have to fill that 100,000-vehicle Amazon order, and I'm not sure they'll have the rate of production high enough to do more than fill their contractual obligations for some time.
It would be great to see Rivian produce chassis for automotive upfitters, so we could have things like electric RVs and ambulances, etc. It may be that
GM's BrightDrop vehicle get this role first.