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A sample plot I always throw out to newer DMs is "Vampires". Its tyhe sort of plot that writes itself, and your players immediatley will know most of the tropes. I'd also assume you've seen enough revisions of the "Vampires" plot to know a wide variety of complications and reactions to throw out there. 

To expand, the PCs are adventurers and stop at an inn in some remote village.  While resting for the night, someone runs in and says something like "He has struck again, oh my poor anastasia has been taken". Turns out this town has a creepy castle 3 hours to the north where a vampire lives. He comes down every so often and eats people's kids.  The towns people would be super grateful if the PCs dealt with it, they noticed all their gear and assume they are adventurers. 

Now your PCs can haggle with the townsfolk on pricing or terms for saving them (Or maybe they do it because they are honorable?). You can safely assume your PCs investigate, Spoiler: Show While traveling to the castle you can have some crazed wolves attack as they travel the forest.  Next they probably need a way to break into the castle. Let them guide it, and just throw some obstacles as they come up with it.

• "We need to scale a wall and go in a window" 
• "We need to slip in through the main gate when his carrage goes through" 
• "We need to wait until daytime"
•  "Lets just bust in the front door"
• I bet he has a secret cave entrance nearby, lets look for it. 

All are viable options, and you can't care which. Let any succeed, but give all an obstacle. climb checks, stealth checks, time delay, strength checks and obviousness. then just describe some creepy castle rooms,  and throw some zombies/vampire spawns in the castle. Maybe the final boss before the vampire is his vampire brides. 

When it comes time to actually kill the vampire forever, just let them discuss the various methods they have heard  to fully kill one, and pick 2-3 that sound cool. Ask for a religion check and whoever beats an average DC knows that those are the ones that work in your setting. 

End of scenario, the PCs now have a super grateful town, who are willing to let the PCs keep the haunted castle, and would love to keep them nearby in case those werewolves [really whatever you want] the vampire drove out return... 

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End result is you have two different campaigns, but only have to do the prep work for one. The first group might decide to do the work for free for the town, but waits until daylight, spending the night looking for rumors and clues, then they slip in through the secret cave. The second group might bend the towns people over in order to take the quest, but then bust in the front door. 

If you feel comfortable adding some complications, the towns people don't know who the vampire is. They just know that someone is coming and sucking the girls dry. There are two probable suspects, the dark and mysterious new miller who only works his mill at night, or the dashing mayor of the region who lives in a nearby castle. Elected mayor because he drove out a tribe of werewolves. the obvious clues point to the miller, who has a skin condition keeping him out of the sun or he breaks out in a rash, but some more subtle clues can lead the mayor. "Sure he's handsome, charming, and everyone loves him, but there is something unnatural about the way he looks at me" says one of anastasia's classmates (Or really any young girl you can find a way to get the PCs to talk to) Everybody in town is pretty sure its the miller but none can prove it, and the PCs can hear all sorts of wild stories about him. You can drag the investigation out as long as your players are interested in it. Could be 20 min, could be 3-4 sessions.

I have a campaign that I ran at Tabletop that I'm running online in play-by-post. One of the players is in both games. The reactions of the party members, the action of the players, and a variety of other events results in having a major difference between the campaigns. In one version, they turned the villain of the story over to the city council for punishment. In another, they turned the evidence over to the rival theives' guilds. The first ended in a trial and exile of the villain into the barbarian lands. The other resulted in a guild war, and the villain returning as the party was one of 3 groups trying to hunt him down and end the guild war.
In one, the party gained acclaim and recognition. In the other, the party was viewed less favorablly as schemers and politicians out to manipulate the fate of the kingdom.
This changed who would hire them a bit.

I also have a cult of necromancers running around, trying (and failing) to invoke the name of the one that inspired them, and each member working to be the true successor to the great Necromancer King. Heh. Loosers are worse than the whiney Sith.

I'm building a quest like that and the way I'll build it is... ( In fact, I might have more than 2 groups )

[Quick Overview]
One group will start South East of the Kingdom and one will be North West of the same Kingdom and they must gather people across the Kingdom to help for a ritual! In a year ( In-game ) the ritual will start. I'll try to keep the groups at the same pace ( Slowing the progress of a faster group ). So, in the end, I'll add up every people they gathered for the ritual and see if they have everyone they need! Then, the quest will be over!

This is the way I'll try it...Hope it will work!