[quote=@June Billson]
A One Piece RP. We can focus on a single crew's adventures around the Grand Line or a special marine operative team who rise up through the ranks, aiming to go to the New World. 
[/quote]

This is the one I'd be most interested in. It's a cool world to explore and there's an inherent freedom to the idea of being a pirate in the One Piece world that would interest me. The separate islands make for easily defined story arcs as well.

[quote=@June Billson]
A My Hero Academia RP. I'm open for a story revolving around students in a hero academy, vigilantes of a certain city, or a group of villains (we can talk about their goals later).
[/quote]

You'd probably get the most interest from something like this, since it's the new big shounen for a lot of people, but I think that interest can be good and bad. I've seen more than a few MHA RPs die really early as a lot of people gain and lose interest quickly; you'll probably get a lot of applicants and 'flash in the pan' interest and then most of them will flake.

I have a character for the setting that I'd love to explore more though, so I'd be one of those people applying.

[quote=@June Billson]
An Isekai RP.[LIST][*]Reincarnated into monsters, so a plot similar to Re: Monster or The Time I was Reincarnated as a Slime. To avoid the grind, we'll have time skips in between and focus on story arcs to keep the plot progressing.
[*]Reincarnated as your ideal self or just brought to a new world as yourselves. You'll be the proclaimed heroes of that world and are tasked to protect them from the incoming demonic onslaught, or something similar. So a plot similar to Shield Hero.
[*]Brought to a fantasy world as an adventurer. So most activities will revolve questing and the like. More semi-sandbox with the occasional plot inserted from time to time.[/LIST]
[/quote]

It's not impossible; the longest running RP I've ever been in, which is still running in fact, is an isekai RP. But it probably takes more of a guiding hand than you're expecting. Giving people free reign over their levelling up and skill gains won't end well; ideally you'd give people a setting and a system to work with and they could go from there or you could be more involved and control when they level and what they have access to.

Maybe think of it as running a DnD campaign without the dice rolls than anything else; you're the DM/GM and they are the players. You create the world and they explore it.