On a site I was on previously, they made use of a thread that members posted links to their open threads, until they were claimed. This made sure there was a singular place where people looking for threads could go to find them.
In addition, if a roleplayer wanted a certain story arc for their character, they'd fill out a Wanted Ad. This way players looking for threads could go through and respond to the ads, sometimes with brand new characters that caused the world to grow!
Just some food for thought.