Sure, a sequel almost always has a fighting chance if the right people are involved
but some films just don't need sequels and the types of stories the Coens tend to tell resolve themselves so satisfyingly, it seems strange that they're even considering it. Yes, there's the insane Lebowski cult which nobody could have predicted but the Coens don't seem like the types who would allow that to influence their creative decisions. There's also that quality that the original "Lebowski" possesses which is one of the main reasons for said cult- a classic case of a film where everything falls together so perfectly, practically every scene is a mini masterpiece- which impossible to produce intentionally and even *more* impossible to REproduce.
I'd probably rather have "Old Fink" than "Lebowski 2: Zombie Donny" but aren't too optimistic about that one either and feel like it's a similar case where they should just leave the original film alone.