Unless they can get a top defensman in a trade, Karlsson is the only free agent I would be interested in, and reports are he wants to stay on the West coast. Otherwise, I'm okay with Sanheim, Provorov, Ghost, Myers, Hagg and Morin. At this point Provorov and Ghostisbehere are veterans. Frost makes the team next year, Farabee and Ratcliffe the year after. Rubstov only played 14 games (he looked really good) with the Phantoms before season ending shoulder surgery. I imagine he will start next year with the Phantoms, but could be with the Flyers by mid-season.
Again, unless they get a top d-man, I'd go after a top line forward as a free agent, Panarin in particular. Stanley Cups are usually won by teams with a superstar player (Caps, Penguins, Blackhawks). The Flyers won't have any superstars, so they will need to win with depth of talent and great goaltending (like the Kings).
2019-2020 roster projection:
Forwards: Giroux, Voracek, JVR, Couturier, Konecny, Frost, Lindblom, Patrick, Laughton, Hartman, Vorobyov, Free Agent
Defense: Provorov, Sanheim, Ghostisbehere, Myers, Morin, Hagg, Gudas (trade McDonald if possible, or send him to the Phantoms to take his 5 Mil salary off the books). Morin probably needs to play a bit in the AHL and see if he can stay healthy and if he proves he is worthy of the call up, they can probably trade Gudas. Schlemko is probably the 7th defenseman in the interim.
Goalies: Hart, Talbot or Elliot