When trying to open the main assembly of a project, sometimes it's difficult to track down the needed main assembly in the midst of a forest of assembly files. Especially when the assembly files were created by others and they use a differente naming convention.
This feature is already implemented in Solidworks, I think.
To avoid spending to much time browsing and opening assembly files, only to find out it's not actually the main assembly, Creo could save in each assembly a flag to indicate if it's a top level assembly or not.
The procedure would be as this: when creating an assembly, set the flag as a top level assembly by default. Whenever an existing assembly is assembled into another assembly, automatically flag the assembled asembly as "Not Top Level Assembly".
When browsing for assemblies, include a "check box" in the User Interface to filter down only "Top Level Assemblies".
The only caveat I see with this suggestion is that the Assemblies when inserted into other assemblies would have to have the flag changed, ie, weitten again to disk. I think it's not problematic when not using Windchill, but could imply a check out if using Windchill. So, it could / should be implemented only if not using a PDM system.