My issue is less coming up with ideas or inspiration and more the actual practicalities of turning that into an adventure. @TheAlkaizer ’s resources look really interesting. Though both do seem to be specifically about dungeon building. Rather than other types of adventures. I might try and find some flowcharts.I might take an idea from a PC. Perhaps he has a history event or ongoing feud or such. I had one PC who's background had him owing a debt for freedom and after several levels of adventuring, I made an adventure with a collector coming around and offering to cancel it for a specific mission.
Sometimes I take the PC's level and find a monster I want to use. Then I make an adventure around the monster. Maybe 3rd level PCs are traveling between villages and I want to have them run into a manticore. There could be a planned encounter where the spot the manticore and maybe a chance it attacks. The players can decide to take the hook and I might have the lair in a 5-room dungeon.
I might also make an item I think one of the PCs would take and place it in someplace they can find it. I add hooks to see if they bite.
Ah. You're right. I've been mostly focused on single-dungeon adventures lately so I equated the two.Though both do seem to be specifically about dungeon building. Rather than other types of adventures. I might try and find some flowcharts.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.