I feel like there could be quite a few top 6 C's made available this off season. For teams like the Blue Jackets and the Wild it might be easier to find those pieces than normal.
Top 6 C potentially available in a trade:
Ryan Strome: Still inexpensive and producing - Rags might want to keep him until one of the young guys proves a little more
Dylan Strome: Maybe losing his top 6 luster, but a team with wingers and a spot open on their top 6 C this could be a buy low option.
JT Miller: Has shown that he can play top line C with Pettersson out this season, good on the draw
Eichel: rumors have it he wants out - expensive but the best pure C potentially available
Reinhart: Hinted he might want out - played primarily wing in the NHL but had a decent year at C this season and played only C in junior
Monahan: Calgary will make some changes, unfortunately for CBJ or Wild they probably need a C back
Kuznetsov: Capitals have expressed some frustration with him, rumored he might be available
Duchene/Johansen: Sort of been there done that for the Jackets and at their cap hits there may not be much interest
Couture: The Sharks would be wise to move on from some of their long term contracts. This is one that might get a return from a C needy team
Hertl: If the Sharks are going re-build, re-tool, re-set, re-what-ever he may be an option, but given his age and contract I imagine the return would be high
Valardi/Turcotte: This is just my opinion - given their young C depth the Kings may consider moving one of their future stud C's for the right return. Unlikely, but there is potential.
Free Agent C's:
RNH: I see him returning to the Oilers, but he is the best "free" C option out there this year IMHO
Krejci: At 35/36 might be getting too old to shoulder a #1 C role but he had a pretty good year this season so he will have some interest from around the league. His age prevent him from being a teams long term solution
Danault: More of a #3C or a #2Cb (play on the 1A/1B goalie designations) but has played top 6 C for the past 3 seasons in Montreal. Not the prototypical top 6 C but an option that only costs money not other assets.
I'm sure there are others and of coarse not all of these players will be traded, but it definitely feels like there are options out there this off season.