I'd like to toss my vote in the hat for "different thread for each LN" but mainly for going forward (start with NT 7). Maybe a rule that discussing stuff that hasn't yet been translated should be behind spoiler tags...
NT 5&6 go together and are recent enough so they might be able to sustain their own thread as well.
For older content... I haven't been reading the LN thread as much as others so hopefully someone else can come up with a better way to divide them, but perhaps:
-one for LN1-13 (basically all that has been animated).
-one for the Side Story novels (but any new SS novels would get their own thread).
-one for 14-22
-one for NT1-4.
Basically it would be nice if someone reading through the novels didn't have to read through all 30ish to begin discussing it without fear of being spoiled (personal bias: I had read through all of them so I could read the LN thread
). Separating the threads would help... kinda like landings on a staircase.
Edit: Also a generic separate spoilers and speculation thread for not-yet-released LNs... sometimes a leaked novel summary turns out to be spot on accurate.