What if each Class had a brand new skill line added to it, with thematic abilities, but we could still only use 3 Class skill lines at a time? To access the 4th Class skill line, we would need to Subclass into our own Class. Other Classes could also Subclass into it; it's not mutually exclusive to your Class.

Said 4th Class skill line could contain skills that would better flesh out Class Identity, as well as round out their toolkit. For instance, if you play a Class which has an undesirable skill line, you can replace that skill line for your Class's 4th skill line, for brand new abilities thematic to your Class.

I think this would help Pureclass players to retain more of their identity while remaining competitive, as each Class's 4th skill line would give them new tools to work with. It would also encourage them to Subclass, albeit into their own Class. Plus, maybe with the addition of a 4th Class skill line for each Class, we'll get a 4th Class Set from the Infinite Archive?