Show off the concepts of Trials and Totems, but at most only show one of the totems and trial captains so we have a basic understanding. Hint at the idea of Grand Trials but don't expand on it. Don't show us almost all there is to know about most of the trials.
Show off the concept of Z-Moves by having the starters use the generic Grass/Fire/Water ones. Don't show us any more.
Show us a bit about the Festival Plaza so competitive players know something about how online battling will work. Don't show us the Join Avenue-like features of it.
Don't show off Team Skull or Aether Foundation.
Don't show us footage of most of the in-game locations.
Don't show us Red and Blue, lol.
Actually despite what I said above, I guess I would say don't show us Type: Null or the Ultra Beasts. S/M were different from past games in that they gave more Pokemon plot relevance. So maybe I'll amend my statement and say just show us all the regular Pokemon that we can actually obtain on our journey and hold off on the ones we can't even use until the post-game.
So in that case we'd know most of the Pokemon and ideas of what's new, but most of the game itself is still a mystery. That would be my ideal pre-release period.
You were a little too harsh but I agree with most of your post ---