The parametric timeline view is great but it suggests another, literal timeline that would be extremely handy.
You could in principle already achieve this with the timeline by dropping parts into an assembly in a specific order and putting them together that way (which incidentally is still a slower and more frustrating process than it should be), but it'd be great to be able to just create a big project (like «Model S»), build up everything from the ground up, and have an easy way to coalesce independently manufactured components into their own single point in the assembly timeline (that way, a user can drop in «steering wheel» as a single point in the timeline but drill into it and make changes to the details of specifically that component or its interfaces to the rest of the assembly when needed).
