ideas that form a gravitational force: these are ideas that pull many, uncoordinated efforts. It is a bit hard to describe, but I think of this as ideas that are "necessary" in a grander emergence that needs the ideas to cement its existence.
Ideas that are not ready to be hatched: "the too early" class. Sometimes because a missing ingredient has not been identified or not sufficiently widely available. When those ideas fail, and a decade or so later new thinkers think them again and realise they failed, sometimes they go back and figure out the prerequisites weren't there and now they are or they build the prerequisites themselves.
Ideas that are non linear: those are the rarest to come by. I believe they require extreme tolerance for counter-factual thinking either naturally or otherwise-inhibition-reducing aids.