The issue with high levels is
Game designers, Adventure Writers, and Monster Makers design like high levels like low levels.
If there is one thing I would ever design first for D&D, it would be a high level group book.
- A list of 5 villainous groups
- A Fighter Army
- A Thieves Guild
- An Evil Church
- A Super Mage
- A Monstrous Group
- Their Lairs
- The high level fortress
- The high level hideout
- The high level temple
- The high level mage tower
- The high level den
- Their members
- Their Leaders
- Their Lieutenants
- Their Henchmen
- Their resources
- Their magic spells
- Their magic items
- Their skills
- Their gold
- Their allies
- Their reactions
- How they deal with teleports
- How they deal with summons
- How they deal with healing
- Possible contingencies for wishes and miracles
- How they can crank up if pressured
- How they can crank up if given downtime
So when the players do X, there is an index in the back to tell you what page to turn to describe how the high level enemies might react.