Tolmie said a while ago that Weigl and Fred was an option, so I don't think we can rule out 2 central midfielders who predominantly want to play as the #6 arriving in the summer. Fact is Fernandinho's getting old, and he's not going to get a summer break. It would not be surprising if he had a drop off like Yaya's post-2014 world cup.
Ndombele + De Jong would be an ideal pair of targets IMO. In Ndombele you have someone who's really good at holding on to the ball, more mobile and more of a genuine box to box, and is probably ready to play right away. In de Jong you've got someone who needs a bit more coaching to know when to go for it, and when not to, but he's got an ability to see passes, or see how he can get a teammate into space with a run is incredible. They both offer the 2 different styles of player you can go for at the back IMO.