I don't think there is anyone that is absolutely going to be available. There are reasons that Tyler Johnson and RNH could be available.
I doubt the Rangers would trade Hayes, but Namestnikov might be available. Wennberg has been disappointing this season in Columbus and Jarmo likes to be bold. It's just whether you believe those guys are 50+ point centres going forward.
Outside of that it is probably players you hope to convert to centre, like Strome or Galchenyuk.
Hopefully Boston can beat Tampa, make it more likely that Tyler Johnson is available.
Is there any 1-year band aid that could tide us over until next years UFA crop, or at least until a deadline trade and sign. There are some really good UFAs that may be available next year, and I doubt all of them will re-sign. Couture, Pavelski, Duchene, Seguin, and Brassard. Then there are some less good options (lower-end, wingers who can play center and older players) like Dzingel, Marcus Johannson, Anders Lee, Henrique, Gourde, Staal, and Spezza. Many will re-sign with their teams, but a few could be available.
For example, if Thornton is healthy, doesn't retire and the Sharks don't want him back, we pay him a big 1-yr payday and acquire a Top 6 winger in trade for Sobotka+ WPG 1st + Prospect. For this example, I am using Hoffman, but put who you want.
Fabbri - Thornton - Tarasenko
Schwartz-Schenn - Hoffman
Steen - Thomas - Berglund
Sosh - Brodziak - Jaskin
Then trade 1st + other good prospect + B prospect + 3rd for Duchene (in this example, but use your choice) at 50% retained, immediately sign him to a 6 year deal.
Fabbri - Duchene - Tarasenko
Schwartz - Schenn - Trade Pick-up
Steen - Thornton - Thomas
Berglund - Brodziak - Jaskin/Sosh
After the playoffs, Thornton retires, we have Duchene-Schenn-Thomas down the middle and can slide Kyrou/Thompson/Kostin into that 3rd line winger role.