One of the most use classes of MediaWiki core are the Title class and the User class. Currently, they depend on each other. Removing this circular dependency is an important step in modularizing MediaWiki core.
Top priority should be given top publicly visible direct dependencies, but eventually, all dependencies binding the two classes together in a circular way should be removed, including indirect dependencies, and dependencies in private code.
This is one of the outcomes of T206080: Wikimedia Technical Conference 2018 Session - Architecting core: layers/components/libraries.