Very interesting thread.
In my most recent campaign I had some vague plans at the start for its structure and who the BBEG would be which morphed, mutated and basically got thrown out by the end. The final confrontations were with bad guys who didn't exist until session 6 and 7 (of 20). It was a superhero game, so the players choice of action was fairly limited - when bad people are doing bad things, the PCs pretty much have to stop it. The morphing mostly took place as a result of which ideas seemed to work best, which I found most interesting. Sometimes an idea seems productive at first but turns out not to lead anywhere. A fair part of it was probably related to player interest but a large part was probably also due to my vapid, flibbertigibbet, easily-bored nature.
I think it is possible to do foreshadowing, in a sense, without having a fixed future, by re-using and developing upon characters and situations that occurred earlier in the campaign.
For example, in session 6, a group of metahumans, calling themselves Metaforce, approached the PCs warning them that the US government would send Executive Sanction, CIA super-assassins, to kill them, just as they had tried to kill Metaforce in the early 90s. In session 10, Metaforce go public, creating an artificial island in the Atlantic and declaring it an independent separatist metahuman nation. Over the next 10 sessions the PCs have various dealings with the inhabitants of this island, there is a lot of tension between the island and the world's governments. Finally in session 20, the PCs narrowly avert all out war between Metahuman Island and metahumans loyal to the US and Russia, including Executive Sanction, who the PCs only met for the first time in that final session. Then they kicked the crap out of the demon-god secretly responsible for stoking the conflict.
Looks like good foreshadowing huh? The thing is, it wasn't planned. Metaforce were just one group among many. I was constantly presenting new characters, situations and places (I wouldn't necessarily call them hooks, because usually they don't have a long-term plot attached when they first appear) in the course of the campaign. Some became important, some were never heard from again. And I had no real idea which were which at the time. It was only around session 16 or so that it had become clear to me that Metahuman Island vs humans would be a major campaign conflict, and even then it wasn't necessarily going to be the campaign's end point, I had two or three other ideas available.
As another example, an NPC superhero called America (a Captain America type) first appeared in session 3. I had absolutely nothing planned when she first showed up, I just liked the character concept. America turned out to be a 'stayer', making numerous appearances until the end of the game, almost always on the same side as the PCs but not always seeing eye-to-eye with them.
On the other hand at one point I introduced a team of superheroes called the Optimates, heroes-for-hire but more corporate, run by a Donald Trump type. I thought they would be an interesting idea but it turned out I couldn't think of a single thing to do with them. So they never showed up again until a brief mention in session 20.