I have an answer-chaining question in the sandbox, but I still haven't come up with a winning criterion.
What scoring criteria are there for answer-chaining questions that encourage users to collaborate for a longer chain?
This is (to my knowledge) basically the "standard" for answer-chaining. Essentially, the overall goal of answer chaining is to make the chain as long as possible, but in order to declare a single "winner" you want to make a condition that:
A simple win condition that accomplishes these is "The winner is the second-to-last answer after no new answers are made within [timespan]". An impossible to follow answer cannot win, a user cannot win if they stop contributing, and there isn't any upper bound for total number of answers, nor a chance for a "soft bound" where posting new answers becomes undesirable (e.g. because the scoring makes it impossible to catch up to existing challengers). An example is the Add a language to a polyglot challenge which has been going on for over a year.
On the other hand, as discussed in some other cases (such as here), using this criterion as an excuse to make a challenge continue forever without a winner is more like a loophole in the "challenge must have a winning criterion" requirement. I haven't seen any better options that can simply be applied to any answer-chaining challenge though.
By the nature of this type of challenge you don't get a "winner". The person who breaks the chain loses which already encourages collaboration. This is probably a good argument for why this type of challenge doesn't quite fit on the site and why the existing winning criteria feels wrong.