Building on this, perhaps an entire sourcebook that is aimed at high-level play, including a few short adventures that could be plugged into different campaigns. I would be more interested in running high-level games if there was more direction given on how to do so.Epic destinies. Hell, I'll expand that- high level material that isn't an afterthought in general.
I forgot 5e didn't have this because I still use itThe Bloodied condition.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.