Yay I am super excited to get started! 
Also I had kind of an Idea to make things a bit more interesting. I'd love if you'd hear me out, of you don't want to do it that is fine too.
Anyways here goes: Intrigue is a very important part to these kind of roleplay, we all agree on this I guess. But how can we really have that, when we know who the foe will be? 
The answer: Minor Houses. Everyone ruling will not have entire control over his realm as he needs subjugates to do that. Minor Houses, Generals, Admirals, Priests and so on. I'd say everyone writes up two or three of these and they will be played by the others. The trick is that for each time the Minors act it will be a coop between the other players via Piratepad, and the people playing them will only use the name of that house, so no one will know who plays whom. To control your nation effectively you will need to trust at least someone, but what if one of your minor houses is controlled by your mortal enemy? And plotting against you? Should get interesting that way.