Not going to happen, but I think De Jong and Casemiro would be an almost ideal midfield 2 in a 4231.
I think Eriksen and Casemiro would be our best midfield two for most games, with what we currently have, if we want a midfield two to allow Bruno as a 10. For tougher games, I guess Fred and Casemiro if we wanted to be solid and playing more on the counter attack, but I don't think ETH would want to play in such a way. So, ideally we need a deep lying playmaker to make a 4231 really work, but its probably our best option at the moment even without a new signing.
Assuming we stick with this 433, with one 6 and two 8s, then unless Bruno can quickly redevelop himself I really don't see him playing as an 8. He's just far too reckless, and whilst his high energy pressing higher up the pitch is good, in the centre he's just too weak and positionally inept. So, that would leave Casemiro, being flanked by Eriksen and DVB or Fred. Neither of those players are individually great, and I don't think Casemiro will fulfill the "connector" role ETH likes, so I don't think this is our best option.