I have some ideas that are open to small groups or 1x1.  I added 18+ because once you involve violence and more mature plot ideas it is best to be on the safe side.

Hoping at least one idea will catch someone's interest:
1-
They had burned them, drowned them. Killed their friends and family. Witchcraft fell away into the shadows for a time. New age, modern witchcraft isn't quite the same as what they once practiced but it is enough to tell them that the world was more willing to overlook those on the fringes.

Now it was time to find them. The grandchildren, great grand children of the ones they killed. Not the falsely accused, no the real witches. The men and women who once did what they could to help.

It was time to form the coven again.
---
So premise is the gathering of witches. They would be men and women who have no idea they hold any sort of real magic. Good or bad is up to the player. Those who are gathering the coven just want to bring them together but things will happen to cause tension...outside forces, etc. Plot points still being worked on and up for discussion with those involved.

2 -
"Adventurers wanted!"

One of so many postings on the town board. Why this one appealed to you is a mystery but you grabbed it and followed the directions to the meeting spot.

An odd group sat near the fire in the tavern and though it was crowded you knew they were who you were looking for.

A woman, dark haired and pale skinned dance to a song only she could hear while two men sat in chairs, talking. Another figure stood in the shadows near the chairs, you can see the glint of a small blade.

"I knew you would come!" The woman has stopped dancing and is looking at you with a dreamy, far away look in her eyes. "The stars whispered to me about you."

One man stands, "Welcome. Please come and sit. I think we have some things to discuss."

The other man watches you. He seems to be taking a measure of you.

You sit and hear what they have to offer.
---------
So fantasy, dungeon crawl, adventure. Pretty much the basics. The group is on the hunt for their next treasure, next adventure and willing to take on whatever comes their way. 

3 - Road Trip

Maybe you are running from something, searching for something and decide to pick up a hitchhiker. Maybe you are the hitchhiker.

---
Few ideas for this one but really depends on who the other(s) involved are.

4 - An island needs exploring.
Think pirate, be it your standard or maybe steampunk. Genre is open but idea would be for a group on a ship charting unknown islands/lands.
---
I have other ideas but really just looking for a person or people to work on a story with.