I'm sure that this has been reiterated multiple times at this point, but: RPs die. That's just how it goes.

After a certain point you want to sit back and look at several things: 

1. What are people currently interested in? What are some popular RPs that are alive right now, and what do they focus on? Do they line up with anything I'm interested in?

2. If the RPs themselves don't interest you, but some of their themes do, utilize those themes in your next thread!

3. If you don't have partners or friends who can/want to join your thread, post an interest check and let people engage with the idea there. 

Realistically, you should also try and join threads if you want to make them. Joining other RPs allows you to meet people, and make friends who then might be interested when you start up a thread of your own.