I hope the level can be elevated rather than him being dragged down to a lower mean - at the very least, he can marshal the midfield and be the general in a manner we haven’t seen in a literal age. There’s a lot for every single midfielder here to learn from him - he tells them to go here or there, and they do it, should be the theme. It’s different to the players you’ve listed outside of Schweinsteiger because of position. This is one of, if not the most, catalytic positions on the pitch - a central hub that so much goes through and is governed by - without him, Kroos and Modric don’t even get the ball; without him the CB’s are exposed, and so on and so forth, so it’s exactly the same for us and he should be seen as a vital conduit because, as the player he’s coming here as, he is.
A spine should have at least one point of governance straight through its key points GK, CB’s, CM’s and CF - if you’re an elite team, AM, too - we’ve seemingly got two at the back now and Casemiro should be precisely that in midfield. We’re obviously short at CF (you mention Ronaldo, but I am not sure at his age and with the frequency of games), but it is still one hell of an uptake for a summer.
Not sure what others expect from him, but for me, he has it in him to be utterly transformative for this team - the dictating central midfielder is one of the strongest threads throughout the history of the sport and I expect him to be one of a long line of them that turns a team from one thing into something wholly different.