Something you can do is make a post like this, asking for interested parties to join what you have planned.
What I've done, is build my arc up for a while with little hints being dropped here and there pretty much since the RP really took off. Since it's character focused, I hate to build the IC relationships so people would care about what was going on, and want to act when things started happening.
Now I'm not saying take three months to build things up, but you need to plant the seeds as you RP with people most times so that they get invested without realizing it and want to join whatever it is.
For something more sudden that involves a villain that say, shows up out of the blue? You can still get the buildup and participant base by making RP threads and testing the waters. The success of those threads is dependent on a variety of factors, but the important thing to keep in mind is that you shouldn't be discouraged by low numbers at the start. As long as you begin the plot/arc with little things and mention it here or there in other RP threads people that are interested should show their interest in a matter of time.
Since my participant base was garnered as the discord servers formed I don't really "plan" the way the arc goes on there, but use it to talk with said participants about times that work well for running threads, and getting misunderstandings about specifics sorted out and asking questions without needing to leave OOC messages. Discord isn't a requirement, but it helps organize things better once interested people are assembled.
