(The whole donkey sex scene is kinda missable, not that I want to, y’know, judge anyone’s interests.)

But I’ll add another nomination for a violation of your rule, and it might surprise you, because it somewhat surprised me. I saw Rocky Balboa last night, and while I’m neither a particular fan of the earlier films, nor hostile to them, I thought this one was really quite good, and both charming and authentic, while still doing the whole boxing thing surprisingly well, in a way that went quite past any of the earlier films, including the original. 16 years since the previous one. I’m not a boxing fan, either. Good characters, though, and Stallone seemed to have almost reached a kind of Eastwoodian iconicity. As I said, I was surprised to think that well of it.

I suppose that I’m going to have to rely on my get-out clause, i.e. that I used the phrase “rule of thumb” rather than “ironclad certainty.” It’s not impossible for a belated sequel to work out well, merely deeply improbable.

Batman Begins wouldn’t count IMHO, since although they’re using some of the same characters the writer/producer/lead actors had changed completely from the 80s/90s Tim Burton/Joel Schumacher films.

I suppose it’s a matter for debate whether you regard the Guber & Peters-produced films directed by Tim Burton as being a distinct series from the pair directed by Joel Schumacher (Tim Burton had a producer credit on the third film, but from what I remember at the time it was clear he wasn’t actively involved in the later films.) Either way, I don’t see the Christopher Nolan film as being a sequel to the films from a decade earlier. (If anything it was a prequel to those films, at least in terms of the internal timeline.) I’d call Batman Begins a new interpretation of common source material. (The same could be said of Bryan Singer’s recent Superman film.)

Films that are explicitly marketed as sequels, like the numerous horror episodes in the Halloween and Friday the 13th sagas, count as a single series in my book despite a huge turnover in actors and directors. The forthcoming Hellboy 2 is part of a series, because the director and most of the cast are returning. The Star Trek films through to Nemesis were a single series, but the new one J J Abrams is working on isn’t part of that series because apart from a cameo appearance by Leonard Nimoy there’s no continuity of cast or creative team. The Tom Cruise Mission: Impossible films are a series because again the main stars and the producer (i.e. Cruise himself) are still on board.