It comes down to who you want to build the team around. Carney is integral because he is an elite playmaker on a long term deal.
WG has stalled in progression due to injury and presumably Asada concerns. Still has excellent potential upside.
Fifita is an attacking weapon and as such is vital to taking this team from a grinding 5th-6th team to a 1st-4th realistic shot at a comp team. On the leadership front, he seems to have a charismatic personality that can influence. A good coach will harness that and make it work. If he is disgruntled it will go pear shaped quickly because he doesn't exactly have a poker face.
The signings of Lewis, Gordon, Heighington and Ryan last year has got us in a bit of a bind. All are experienced and at or slightly past their peaks. They were bought to provide a 2-3 year premiership window with a high quality back being the missing ingredient. Asada stuffed that a bit with the uncertainty over coaching and some high profile players.
Potentially losing Fifita or WG and keeping Lewis and Heighington etc creates balance problems with a bunch of key forwards getting old together shortening the window.