If we were just looking to potentially upgrade what we have to play a CM2, there are cm's I really like, like Sangare (PSV), Koné (BMG) but if it's to play a CM3 and we already have a bunch of 6's now, assuming we'd really want someone to be an more defined 8 type (ie more biased toward progression/attacking/creativity) then I guess people like Bellingham, Majer, Aouar, Thuram(K), Caqueret, Brandt, Wirtz, Lopez(Max), Tonali, Barella.
I get that some will question whether some of those are archetypal 8's. Some of those are playing as 6 or 6 hybrids so people might query their inclusion, but I think some like Caqueret, could also play the 8 remit well too, as he's got the technical ability and looks forwards always. Same Tonali (Rabiot), probably thought of more as a 6 type, but he's pretty good at progressing and supplementing the attack. Conversely there's people like Wirtz (or Brand), who are probably used more as AM'S, but in a robust CM3 with two strong 6 types, you could easily drop him into the CM3 as the link guy - same way Eriksen did at Inter (and for us).
Just a bunch of names off the top of my head that could do the job, and some could also do other jobs as well, making them potentially good value. Some more viable than others, some flawed but still maybe worth a punt at the right price.