This might be a long shot and I dont know if its technically possible to implement, but I think this would be a major improvement to after effects as a whole.
I think the easiest way to explain it is looking at the dynamic link feature between premiere and AE, but from AE projects to AE projects.
I would have plenty of usecases for this and I it would elevate professional workflows into new heights.
Just like having your own library of functions in coding, that you can link in multiple scripts, updating or adding new logic to the source function which automatically integrates in the scripts where its linked to. In this case the function would be an After effects project that you can reuse and implement in other projects and automatically updates in all linked projects if theres changes to the master project.
Reallife example: I have a beverage flatlabel that I wrap around a beverage can with UV Map. I use this Label in multiple projects, since theres plenty of different campaigns and animations where this can is animated differently and set into different scenes.
everytime anything on the master flatlabel changes, or we have to translate it into different languages for local campaigns, then I have to manually update and import the label project in ALL my different campaigns to update the Label there. If this Live Link would be a feature, then I could just do the changes in the master file and it automatically implements into all projects.
This could also be further expanded in a way like Mogrts, that you can even control certain properties when its imported/live linked from the master project, without manipulating the actual master project.
Obviously this would be a pretty major feature, but Im certain that this would have a huge impact on general user experience and complex professional workflows.