My old school campaign: I ran some basic fantasy a LONG time ago with a group. The big bad was this Hobgoblin and his army. The PCs never quite got to him. They just wandered around doing adventures. Obviously the sessions ended, however world kept moving with or without the PCs. The nagging thing in my head is starting an adventure after the war. The hobgoblin and his troops have devastated the countryside, the forests burn, the dwarven mines have been flooded, there is death everywhere. In the midst of all these chaos the PCs are holed up in the basement of Elerd's Keep, the sounds of catapult ammunition bounces off the walls, creating rumbles in the room. Elerd sits with the PCs. "It's time for you to save the world....." says Elerd. The PCs question how. Elerd, whose pale skin shines in the lantern light points at a small ebony stone altar. "That is a time stone, and I intend to send you back, to right these wrongs and save our world".
(I should note that Elerd is a local ruler, whose been around for years. He is a vampire. But not totally evil. Every few years the local populace starts to figure out that he doesnt age. At that point he marries a woman, has a child, that child then becomes the newest ruler. It's a whole bunch of magic, and sleight of hand to make it look like he's actually human. In reality he's just figured out away to make it work, using doppelgangers at public appearances. )
The PCs are sent back thru time, the goal to stop the hobgoblin. They need to make the Elves and Dwarves aware of the situation. Setup alliances, possibly run errands for them, and be as diplomatic as possible without coming off as crazy. The supply lines for the Hobgoblin army need to be destroyed. There's small encampments all over the northern part of the province. This provides a ton of adventure. At some point the PCs may meet 3 witches, who send them into a demon realm looking for a weapon to destroy the hobgoblins (or some other coolness). There's human barbarians that play a role in the adventure as well, similar to the elves & dwarves they don't care much for the human settlement of Elerds Keep. At some point hopefully the PCs are able to convince them to help.
There's a random mechanic for time travel. Anytime the PCs show up in a new location, there's a 5% cumulative chance that they may run into their previous self. The disasters of which could be catastrophic! One handy thing about this is that if one of the future PCs die, the past PC could be introduced to the party. Of course at a lower level.
There's also a statue in the wilderness that will grant visions of the future. Foreshadowing the hell that has already happened. (yup timey wimey stuff. I love it!)
Eventually the PCs get to the giant battle. It may take place on the steps of the Hobgoblins headquarters, or it may take place at Elerd's Keep. It could even take place around a small halfing village. The fight ensues. The players take over regiments of Elves, Dwarves and barbarians fighting against the forces of chaos. Cut scenes to individual PCs fighting hobgoblins for a few rounds.
Unfortunately this whole thing is a bit railroadey and not entirely old school. But sections and a map could detail different parts to it. Setting it up as more of a hexcrawl, the PCs could make a beeline straight to the hobgoblins palace if they want.
This is one of a few different campaigns/worlds that I've been working on whilst attempting to fall asleep. Hopefully I'll get a bit of rest this evening, before Crimhuck wiggles it's way into my head. (Which is a whole different thing).