To save time and brain power it would be nice to have independent from project macro and function libraries that you can use In any project. Would save a lot of time. Also would be cool if people had ability to download them from marketplace 
You can migrate assets between projects on Unreal Engine
i am using folder junctions, to use the c++ files in different projects.
Migration isn’t a scalable solution. Migration is fine if you’ve developed something you don’t intend to touch again, but is worthless if you need to port changes back and forth between projects.
Or the possibility to make unreal packages, like used on the marketplace, by yourself. A collection of reusable asset (like lighting scenes, basic blueprints, skysphere materials and so on).