I want us to go for SMS, but I think that'll happen next season when Khedira leaves. As for this season, we should try for Nzonzi or Pereira from Porto. I guess since we are looking for experienced mids, others interesting names, who happen to be younger, are definitely out of the question at this point.
A midfield of Marchisio, Pjanic, Nzonzi/Pereira, Bentancur, and SMS to start the 2018-2019 season would definitely be something to smile about.