FreeTheSlaves
Adventurer
My high level experience is limited to 12th level campaigns and a one off 20th level session; so make of it what you will.
[1] Spells, magic items and special abilities need to be looked at on a case by case basis. They may need to better understood, altered or banned; the 3.0 cloak of displacement was permanently on, I didn't like that, so I gave it a duration.
[2] The above can only be done by sitting down with your players and discussing the commonly intended tactics and ponder the internal logic. Afaic something the PC's can do will be done by the NPC's. There must be a defence and MAD is a situation that I won't tolerate (disjunction has a blanket temporary effect or a targetted permanent effect).
[3] The adventure design moves from an encounter area basis to an event basis. Locations are so easily bypassed that their detail can be dropped; special locations are the exception rather than the rule. Events are a sequence of timed encounters that largely can't be interrupted by the PC's so they will 'create the adventure' by setting a scene.
[4] That leads to the proactive v reactive angle. Higher level play allows for a more proactive approach by the PC's. I asked my players broadly what they intended to do next session to help me plan.
[5] Statting up NPC's and special encounter areas required a plan of reaction so to speak. How will they react when under attack? Do they have a counterattack potential.
[6] BBEG lairs must be warded against scry and interdimensional travel. The magical trap construction rules can be used to permanently affect an area.
[7] Important information does not have to be given to the PC's via a lame journal. Their knowledge skills and divinations should be adequate enough.