Flashbacks would have been a stretch, as John Goodman really can’t pass as 20 years younger than his current age of 64.
As for a return as a ghost… well, I’m not even going to try and justify that as a way to go.
Dream sequences could have worked, but they would have severely limited what the show could do with a character.
All things considered, ignoring the finale reveal and bringing Dan back alive and joking is probably the best route for the revival if it was going to incorporate the legendary John Goodman, who has been enthusiastic about a revival for a while now.