So..
Arsenal, Inter & Juve all apparently want Matic too, with Chelsea unwilling to sell to us
Spurs aren't willing to listen to any offer for Dier (they obviously are but they will clearly want a stupid fee)
Bakayoko is predictably going to Chelsea
Any signings in this position that we can pull out of left field like with Lukaku?
It depends on whether we are going for a DCM or a CM and move Ander into the CDM instead.
Out of the CDM's available and the ones you have mentioned above it would likely be one of Weigl(Dortmund), Dendocker(Anderlect), Geis(Schalke) or Fabinho(Monaco). However it seems that Mourinho wants a PL proven CDM and it would mean looking towards Ndidi(Leicester), Romeu(Saints) or Gueye(Toffees) who neither are top tier CDM's.
However if we deploy Ander in the CDM role it widens the options available in the market. I'm all for Nainggolan. I think we could replicate the Pirlo-Vidal-Pogba midfield that really strolled the Serie A. I really wanted Vidal back in the days and I think Nainggolan could be a similar player for us. An all action midfielder that contributes heavily in both defence and attack and has both goals and tackles in him. A player that would invest 100% and play for the badge no matter the costs. If he ever was to leave Rome this year should be it, he wont get a better chance to come to a bigger club and compete for trophies. Roma have sold some of their best players already, so it is not likely that they will be competing for any trophies next year.
The only thing concern is whether Ander would be able to just sit back and be our CDM or if Mourinho finds him too "small" to take up that job. It would limit some of Ander's forces, especially his high pressing nature, but I do however believe that he has the attributes and the brain to transform his game into being a CDM instead.
It does also seem like he is close to agreeing a new contract so it might be pure speculation that he even want's to move.
Other alternatives for the all-action CM would likely be Milinkovic-Savic(Lazio), Goretzka(Schalke), Matuidi(PSG), Seri/Cyprien(Nice).