My gaming group and I just finished a campaign where I was GM and everybody finished around level 18. We had a blast, but the amount of work I had to do to challenge a party that had such high level resources was sometimes daunting especially since I work full time. I had promised to 'go to L 20' but they honestly conquered every foe both in the published material and the ones I came up with, including two gods. A party with high level non-detection, clever hideouts and the ability to teleport multiple times is a tough nut to crack. When prepared, they're damn near immune or close to it to everything. High ACs, saves, healing output and condition mitigation. Of course we have a number of house rules outlawing certain archetypes, spells, Feats, etc.; too many to list here. I have some time before I'm up
to GM again and I'm toying with an idea-to limit the number of magical buffs on a target. Magic is an unstable, disruptive force that cannot exist in the same spot for any length of time before dissipating. Rules wise it would mean that except for spells and effects that are permanent or instantaneous, a target can only hold one same-duration spell of each school of magic at the same time. Magic items would be unaffected, being permanent; except for potions which would stack once because they make anyone a caster. Scrolls would count as spells. Class abilities that aren't spells would be an exception for players and would stack for the individual only. So your own spell-like and supernatural abilities would stack on you. These rules would not pertain to extraordinary abilities at all. Yes, this means that spells that effect a target would also dispel a similar-duration spell of the same school on a failed save. I'm thinking emanations 'centered on you' would also stack once per school. Hmmm, there is much to ponder.