So what I think the plan with him is this:
1) He's going to have that typical first season bedding in period to learn pep's tactics and all.
2) He actually won't be playing midfield as much as we would think he will be. It'll be a bit like Bernardo where he will play on the wings for a while. I actually have a theory that he will play on the RW rotating with Mahrez and learn positional tactics of playing in that Right-wing/Halfspace area. This is because Grealish and KDB are similar in that they are both really aggressive attacking wise so I think pep is gonna teach him the "KDB 8" style role. I know that he has been playing on the left virtually his whole career but pep is gonna pep and ensure he learns how to play in other areas of the position. Also because KDB is not getting any younger with increasing injury occurrences so he's gonna be groomed into that role.
3) He will rotate between RW/RAM positions just like Bernardo did.
4) This also ties in with Phil because Pep sees Phil as a midfielder and is going to eventually shift inside that David Silva role that he's been always talked about to be the heir for. The only reason now that Phil isn't playing in his preferred midfield role is because he hasn't got the "Pausa" that Silva and Gundo have. This is important as it provides a balance to the aggressive midfield playstyle of KDB/Grealish. Other than that he's got nearly everything locked down, aggressiveness, keeping the ball, playing in tight spaces, etc...
But however, that seems to be changing as Phil alluded to this in the Rio interview before the UCL where he said that(to paraphrase) "The manager tells me that I play at a fast tempo and that I always want to attack instead of just passing the ball around sometimes" So he's beginning to understand that it's not always about turning on the ball and going at defenders but to keep the ball moving, keep possession and set the tempo. But it's fine anyways he was and still is young so it's to be expected.
5) Plus with the chance of Kane arriving we could see sterling reprise his LW role as he plays much better as the runner off striker, which isn't something he had last season hence the downturn in form. Plus with Mahrez in great form, it only serves to expedite the aforementioned process above.
6) In summary, it's exciting times, we could see something not only England and Southgate have failed to do which is play Grealish and Phil together, but the possibility of both of them playing together as 8's is what excites me even more. Imagine.. the 2 best English mavericks playing together centrally. WOW.
I could expect to see something like this Initially:
Sterling/Foden/Torres Kane/Jesus/Torres Mahrez/Grealish/Torres
Gundo/Foden Rodri/Ferna KDB/Grealish
Zinchenko/Mendy Dias/Laporte Stones/Ake? Walker/Cancelo
Ederson/Steffen