You should now start to add a previous season mechanic perhaps in an abbreviated fashion.
For example when you hit torment 2 add the vampire zones and such back but only for t2- give it a hook like you have to do coil shut with it to unlock t3
T3 add the witch season exclusively to t3 back do something neat to unlock t4.
And so on.
This gives people reason to go to new torment levels.
As the catalogue of season mechanics grow you can vary them more.
Each season the previous mechanics should be rotated or randomized so it’s not always vampires in season 2.
Also each one should give a specific reward for completing.