The only implementation that comes close to this i could think of would be something i already suggested a couple of times before.
A Trainings Room Enviroment, with maximum skill points and passive points.
You can change skills, masteries and passives as you desire in a specific area with several trainings dummies, maybe even spawn specific mob types that fight you.
Once you leave, everything returns to normal, how it was before you entered that trainigns room.
This feature could be somewhat tied into the “time traveling” aspect, where your hero explores it’s potential futures.
This could be accessible before choosing the mastery in the story.