I'd rather keep the 40-goal scorer who has documented chemistry with both of the team's top two centers and has a reputation for producing in the playoffs.
Rust is the piece to move, but I think it'd be extremely difficult.
If you can't move Rust, you need to construct a third line that carries a heavy load with Rust being the central figure. The only way this is possible is if you find a way to get out from under Granlund and Carter's contracts and go with a decently priced tandem in goal.
Guentzel-Crosby-AA
BB-Malkin-Rakell
CC-DD-Rust
DOC-Poehling-EE
FF
AA and BB should be in the mold of Zucker, guys who get in on the forecheck, are reliable defensively, and can chip in 20 goals by going to the net or surprising a goalie with a shot.
CC/DD need to be in the mold of prime Hagelin/Bonino. Defensively conscious guys who have no problem swallowing tough minutes because they can use their speed and hockey IQ to get out of their own zone and pressure the opposing D.
EE is whichever young guy is playing well with a bit of scoring punch. The fourth line is 25-and-under line. Puustinen, Nylander, Poulin, Gruden. Bring them up and let them go for 10-15 games, see if they stick.
FF is the vet 13th forward on league minimum you can stick in there because of injuries but then bench easily without there being an issue.