I'm hoping that with the next campaign, we will try E9 variant for 5E.
9th level seem like a nice power level to stop vertical power growth.
casters get their single 5th level slot, 1 spell a day of 5th level should not break too much.
proficiency is just bumped to +4, this give nice spread of +0, +2, +4, +8 for various levels of proficiency.
half casters get their 3rd level spells, and subclasses that didn't get any features at level7, catch up that sub class slot at level 9.
from there on, when DM feels that you earned a "level", just gain an ASI/feat. Alternately, you can gain a level in another class(without HPs or moving towards higher prof bonus), kind of gestalting it.