For gaining a large number of players... Don't try to. Its really all about doing something you love, not fishing for how many people join it. I've had many successful projects that collected a lot of people over time. It's just about finding a bunch of people who like to spend time on the servers and enjoying the motive of it. Like HaloRP, there's a good amount of people who would love to Roleplay as marines and whatnot. Loads of addons on the workshop to use too.