Something he developed whilst at Barca watching basketball.
I don't know if you have watched NBA or any league, for that matter, but a lot of what Pep adapted he took from watching basketball. The premise being that in basketball it's cramped with ten men in one side of the court, and so in basketball you need creative runs and passes to create opportunities. It works so well for Pep because, unlike other teams, he implements it intentionally and has his teams practice it perpetually until it in effect becomes automaton.
This is why quality crossing at fullbacks will be so important I think. They can function the way 3pt shooting spacers do to give the stars room to operate. I don't follow team handball as I am not a European but I bet there's lessons from there too.
Anyways, because of basketball, I expect to see a lot of this type of thing in the coming years to force teams to defend the most space possible.
-----------Speedy 9 who can head the ball------------
--Rightfooted Star----------------Leftfooted star
------------MobileCM----MobileCM----------------------
Crosser-----------LongPasser-----------------Crosser
--------------LongPasser----LongPasser---------------
so teams looking like this to take some random names as archetypes and not just the best players at spots:
-------------------------Calvert-Lewin-----------------
---------------Grealish-----------------------Ziyech----------------
--------------------Goretzka--------Doucore-----------------------------
Max----------------------Paredes------------------------Reece James
--------------------Dunk--------------Bonucci--------------------------
but obviously it's tough to find fullbacks and center backs who can provide quality long passing and still do their job defensively and (out wide) run fast enough to help out.
But yeah a team like the above, you couldn't defend deep against with quality crossers at fullback and long-range shooters able to hit a proper aerial target up top in Calvert-Lewin. That crossing ability should force fullbacks and wider midfielders to push up and leave more space for 1 on 1 dribblers like Grealish and Ziyech to cut inside and attack space. Goretzka and Doucoure run around covering defensively and getting on to 2nd balls. The center backs and Paredes can pop balls over the top so you have to get up on them too.
The negatives here are that you'd have to teach Zelinski and Grealish (and really Calvert-Lewin) to make great runs in behind and the CBs, FBs (not James) and DM are so slow you'd be in trouble on the counter.
But my main point is that good teams should move in this general direction to force teams to press up high against them and that's how they can create space in behind. Otherwise you're letting teams sit 10 behind the ball against you and that's the choice for most poorer teams for a reason.