I think, if we're going to focus on a midfielder, it really should be someone who can play as a deeplying playmaker.
Although he's 29, I think Kimmich would be the perfect replacement for Casemiro, and a player who would suit the type of football ten Hag wants to play. Statically, he's one of the best around when it comes to possession based football, ranking in the 98th percentile for both passes attempted and progressive passes. Along with progressive carries, the aforementioned attributes are what we really lack in our midfield. We struggle to control games, simply because we haven't a consistent passer of the ball in the first phase.
Mainoo, despite being so young, is the closest player to that, and in my opinion, we see a difference when he plays as a 6, in comparison to Casemiro, but I think next season, ten Hag will want to continue using him as an 8 and 6, similar to how Arteta uses Declan Rice. Having watched him play in those higher positions over the last few weeks, it would be a shame to put a restriction on his skillset playing him further back, as he is so neat and tidy in the final third, as we saw against Wolves and Liverpool, for example.
Therefore, I think if we have an option like Kimmich, who will be someone we can build the play around, the two number eights, whether it be Bruno, Mainoo, Mount or McTominay, can push into those half spaces and be more of a goal threat. You only have to look at the amount of goals McTominay has scored this season pushing on into the box. Mount shares similar attributes, as does Bruno, as we saw when he first arrived at the club.
Now, some may say he's too old, but he'd provide experience, know-how, ability and leadership. Think back to when Arteta purchased Jorginho. 32 now, but still playing a pivotal role in Arsenal's title charge for the reasons I mentioned above. In my opinion, Kimmich would come in an do the same!
Bruno/Mount - Mainoo/McTominay
Kimmich/Mainoo