You guys insisting on on book "reality" trumping logical/plausible/enjoyable gameplay are going to keep floundering in circles and never get the mod made (even if you had CK2 infront of you now). Its a book, its not reality. Martin is not an historian or Oxford professor of English. He hasnt created entire languages and a legendarium like depth for his world. 'Warden' is the name of a title he read somewhere and thought sounded cool, just like "King of Salt and Rock" and the other pseudo medeivalisms like nuncle, neeps, nonces and whatever the hell else he plucks from the Victorian era or Shakespeare. "Warden" and other parts of Martin's feudal system dont bear such close scrutiny as to be translatable into a computer game.
Why? One of the great moans about Grell's mod (and the reason i never played it past the opening year) was that it was really unrealistic because Lords Paramount had to be King-tier because of the absence of barons. With the addition of two tiers, we can have the Iron Throne as an Emperor title holding the King in the North etc. with the Great Houses as vassals. How is independence going to work in your system? In that system the Starks could usurp the King in the North and go to war with the Iron Throne to claim it. In yours they'll have to slowly lose loyalty and the rebellions will probably quite spaced out, nothing like the dramtic climax in Game of Thrones and probably quite boring for the player.
Also, prestige. Shouldn't a Robb who swore fealty to Renly or Stannis (an Emperor-level title) but nominally remained a King be above the other Great Houses? But not only that, in terms of prestige gain, or anything else apart from scutage, an independent King of the Reach is no different to King of the Reach, vassal to the Iron Throne. In my system you get more prestige, you get to rule your kingdom on your whims alone and you get to be superior in rank to all the other Great Houses.
In yours, you get to not pay scutage.
But there's a more general problem affecting both my system and yours (I've no idea what Nick's or anyone else's is) of how we're going to get the Tyrells and Tullys to swear loyalty to Renly and Robb. In CK1 just because you had a claim on a King title and were fighting to uphold it didn't make you a King. If this changes in CK2 mine will be fine but you'll still be in trouble as the Tullys would be kings anyway and would have no reason to swear loyalty to another king.