Mentioned above but formatting makes a massive difference. If anything I'd say it's the one factor that will kill ANY roleplay if it's not done correctly. Headings that pop, readable paragraphs, and an ease to find information is key. You want people to join so take some time to give it a gloss over in style.

[b]Example:[/b] [url=https://www.roleplayerguild.com/topics/163267-fundamental-rules-of-the-guild/ooc#post-4291608]Site Rules[/url]

Indent tags, headers, lists, and a colour tag. It is so basic and yet it just appears nice and readable. If you have an RP in the works and need some advice for style, I'm only a PM away.

But let's say you have all that then, like [@ArenaSnow] said, use themes that are popular at the time and ideas that people seem interested in.

[b]Example:[/b] Wanna make a Star Wars RP? Then don't do it now...
Create it and hold onto it til December this year.
Why?
Because the new movie—The Last Jedi—will be out, and I can bet people will be flooding over any decent Star Wars RP around that time period.

Lastly, if you do wanna create a unique roleplay, be prepared to repost it at a later date if you don't get the attention you need. I've done it before where I've posted the same interest check months apart and gotten two different results. You only need to look at what happens during 1x1 interest check bumps to see how timing can matter.