VAN could go after Stone in UFA but like someone else said there will be 30 other teams making an offer for him. If Panarin and Stone make it to UFA they will probably sign the biggest UFA contracts since Parise and Suter I could easily see 9-10x7 for Panarin, and 8-9.5x7 for Stone.
Some safer/realistic players to go after that could compliment Pettersson and or Horvat would be: Ryan Dzingle, Tomas Tatar, Jason Zucker, Nino Niederreiter, Cam Atkinson, Brandon Saad (if CHI blows it up), Jakob Silfverberg, Kevin Fiala, Andre Burakovsky. and Palat or Johnson (If TBL go after Karlsson in UFA). I took into account: age, potential, price, and availability in the players I listed, they aren't game breakers or all-stars but they could could provide very valuable support for Pettersson or Horvat.