^ don't think there's any chance of that.
I think Spalletti sees his players as first and second choice versions of one another within his certain system. Namely, his 4231. Most of our players have found their home within that system and I don't personally believe Spalletti is super keen on re-housing these players so to speak.
Nainggolan is the AM (or the most offensive CM) in that formation. I believe that is expressly what he was purchased for, and I don't think we'll see him play in many other positions throughout the season. Likewise, Brozovic has flourished as the playmaker in the LCM position and I can't see him being moved. Vecino has had some success playing as the box to box in the RCM position and I'd say he's considered Spalletti's first choice for that specific position/role. Valero has largely done quite well this season and he's made himself a viable option for both Nainggolan and Brozovic's role.
Joao Mario is the interesting factor in that he doesn't really fit into anyone else's role. Not from what we've previously seen anyway. He's said himself he prefers to play on one side or the other of a 3 man midfield. Kudos to Spalletti, he managed to create a functional 3 man midfield, still getting a good output from Brozovic, Vecino, Gagliardini and Joao Mario in his favored position, and we got 2 great results against Lazio and Genoa. Small sample size, but I think the 433 midfield looked very fluid in attack yet structured in those 2 matches. If Mario can continue to contribute, there's no reason he shouldn't be relied on.
Ultimately I think any lineup and formation will end up depending on Nainggolan's fitness. When he's fit, we'll play our 4231 with him as the #10. If he's not fit, we'll either see Valero as the #10 in the same 4231, or, we'll (hopefully) see Joao Mario in a 433.
After the Frosinone match, we've got 4 massive fixtures, 2 of them in the CL of course. It goes without saying we'll need Nainggolan in good fitness for our best chance of taking points from our direct top 4 rivals, and to qualify from our CL group. Nainggolan was painfully unfit against Barca and he was easily the least useful player in that match. Against Frosinone, I don't care whether Mario or Nainggolan starts, but I want both of them to play.