Just so we're clear.
A One-Shot adventure is a quick and easy adventure that ends with the main problem / villain / threat eliminated permanently. If Lord Blackwater the killer of all things living is dead at the end of your writing and the PC's next game takes them to an entirely new adventure, then you wrote a One-Shot. One-Shots can easily be turned into full campaigns by a GM who just adds some content, Red Herrings, side-quests, and other popular stalling tricks.
An Adventure Hook is something that just gives the group a shove in the right direction. A pat on the butt and a point down the road (so to speak). If Lord Blackwater has been "Thwarted for now" but managed to make his escape with the crown jewels / princess / etc at the end of your writing and the PC's next game puts them back on his trail, then you wrote a Hook. Hooks provide all the makings of a campaign but ultimately leave the fate of everything in the hands of the GMs. The antagonist, the element (town), the macguffin (important item) is all created by you, but once they're given the materials GMs should just run with it.
NEITHER OF THESE ARE WRONG!
You write whatever you want! But it might be good to include in the header of your post to which of these your story applies.
Can we do even smaller sections, like a particular even or encounter that could later be expanded? Like I really liked the idea of an innkeeper that hated adventurers, its something that probably will stir up some role play discussions during the session. Its not a very complete section that you can do much with, but once the PC's find out why he hates adventurers maybe there's something they can do that will change his opinion of them, which could become an interesting adventure.
Maybe this fits in with your discussion on how to add a new adventure to an existing town. These sort of events could be the triggers that spawn the new quests.
Yeah we could do those. But if we're doing encounters we should probably post two at a time so the readers don't feel shortchanged.