@Darkiplier I can understand that--it can be kinda limiting when the GM basically controls everything, it's nice to have some breathing room to do dumb shit if the players want to XD
I myself am not well-versed in the art of group roleplays, I haven't done one in years, all my roleplays currently are 1x1s for the most part. Right off the bat, your idea feels good for a group roleplay, with the four kingdoms and such, people can spread out all over the place. Someone's going to have to make a lot of other characters to make sure everyone can interact with the world, though, otherwise the players will feel kinda left out and not important to the story, or whatever plot you end up forming, even if it's a loose plot. So unless there's like some sort of hub where everyone can meet up, like a headquarters of sorts for the riders, you may need to have a lot of NPC characters on hand if someone's wandering around in a different country than everyone else. Or you could require everyone to have a minimum of four characters--one at least for each kingdom. To make it fair, you'd also have to have at least that many, that way you're giving everyone some attention.
These are just some ideas, you really don't want this RP to die because your players don't feel welcome or important. That would be very sad.