Inspired by classic and building on the current way of how classes and specs work in retail, also, as a way to introduce an ever green multi-path progression system replacing borrowed power systems and legendary gear based abilities, this speculated change will bring multiple paths per spec allowing, for example, a frost mage to further choose a path that suits their play style.
This system would also be replacing PvP-only talents and reintroducing iconic PvP talents as part of this new multi-path-talent tree per-spec progression system that is unlocked at max level - which would forever kill the idea of fourth specs and allow blizzard to focus on creating new classes instead as well as ensure that all the work that goes into borrowed power progression systems does not get invalidated or become obsolete with new expansions.
Coupled with gear sets as the “temporary per expansion powers” giving players a fresh way of playing their favorite spec every raid tier and encouraging players to raid at their desired raid difficulty. This change alone will make WoW great again.
Edit: it might not be multiple trees per spec but a multi branch single tree would be the same thing but visually organized into a single tree with multiple paths.