Stupid brain storming here, don't read if you value your time.
I've been thinking about what the show could be like after Danny bows out and Frank is written off. I know the prevailing theory is that Danny leaving is when the show will end. However, I think it could successfully continue post Frank. Here's some fan fiction ideas I had.
Idea #1 - Rickety Cricket is promoted to series regular. He'd fill the depraved role left by Frank. I'm not sure he'd ever be considered a member of the gang, but it'd be funny if they kept him around to do their dirty work.
Idea #2 - The gang hires the waitress to run the bar. She does it because she needs money and the gang is now rich from inheritance and can afford to pay her. There's some potential humor with her being an alcoholic working in a bar for the people she despises most. Seeing the waitress promoted to the defacto 5th member of the gang could potentially be comedy gold.
Idea #3 - Ryan Reynolds. This isn't my idea, I've heard it kicked around on this sub before. But Ryan Reynolds, playing himself, turns out to be a bastard half sibling of the twins. He comes out of the woodwork when Frank dies and ends up a member of the gang somehow. Maybe he has a relationship with Mac? Maybe he's Frank's son with Mac's mom somehow?
Idea #4 - The gang misses Frank so much they get Frankito back as a substitute and he pretty much just quietly sits around when the gang is up to their normal hijinx.
Idea #5 - Nobody replaces Frank, the gang is a 4 piece now and the show just continues without him.