The way (most) opponents play against Villa is different to the way (most) opponents play against City. Sides defend deeper against City and in more numbers, so City always have a lot of possession and need to break sides down.. either by the speed of attacks (players like Mahrez, Sterling) or by great passes (KdB, Silva when he was there) or a mix (Foden).
City have two holding midfielders to rotate for that position then Gundogan, Mahrez, B Silva, Sterling, Foden, KdB, Torres, Jesus to play the other five. That list has speed, intricate passing, quick passing, individual skill and I'm struggling to see what Grealish adds that City didn't have already. Then add he costs £100m, it makes no sense at all, other than "look at us, we've got a popular England Intl".
Grealish was a very good player at Villa and he suited their game, he suited the way a lot of opponents set up against them (some space to run at defenders) and a lot of stuff went through him. I'm sure he'll win trophies at City but I don't see that he will make a difference between them winning trophies and not.
Kane on the other hand...