N'Zonzi is a very good player, better than anyone we have in defensive midfield. I'm surprised he hasn't moved to a bigger club yet.
Still, I'd rather see us invest in a creative passer or a box-to-box midfielder. Right now, Rooney is the only player we have with any real vision in midfield and Davies is our only true box-to-box player*. Gana (when he's disciplined) and Schneiderlin (when he isn't hiding, see: last season) have enough quality to cover our defensive midfield needs. If Schneiderlin continues to play like he has, I can see the argument for adding a DM, but as is, I think we'd benefit more from someone who contributes something going forward, whether it be chance creation or goals (both, preferably).
I'd like to fit Klaassen into the midfield picture but he doesn't offer enough defensively to play box-to-box and he doesn't fit the passer mold at all. Maybe he has something more to show or maybe Allardyce has another role in mind for him. We'll see. McCarthy showed glimpses of being able to contribute in attack under Koeman but he's never fit, so it's hard to expect anything from him. Besic has a good attitude and some interesting qualities but he's more of a DM and I don't think he'll amount to much without regular appearances (which I don't think he's good enough to earn here).
If we receive a good offer for any of those players, it probably makes sense to sell (as much as I'd like to see more of Klaassen, I'm having a hard time seeing how he works in the context of this squad).
* Edit: Sigurdsson does the box-to-box thing for Iceland so he's a possibility in the middle, as well.