Warning, out of the box:
In my opinion Inventor should offer more integration of multibody parts. My opinion there shouldn't be a distinction whether working in part or assembly environment.
Below is how and why I think so:
Natively functionality to derive and or place a part within a part, as well as derive and or place an assembly in a part, and a (single) body in a part or assembly etc.
Maybe an assembly is unnecessary at all when all assembly functionalities/feastures would be available in the part environment...,
This also means that every single body within a part should be equipped with the same properties as a single part, for BOM purposes etc.
And on idw, it should be possible to place a single body.
When Inventor behaves like I wrote, this should save a massive time-consuming job to generate all those parts, assemblies, assembly constraints, afterwards.
It also could mean a new environment within Inventor, to work in one level instead of a tree structure
