[Anaconda]

The concept sounds interesting indeed. The game politics would be as unpredictable as politics in The Song of Ice and Fire. However D3 game mechanics are not that functional for vassal based feodalism. It nees to be something like Paradox's Crusader Kings, or alike.

In order to capitulation have some significance the subject needs to control more than just the seat of power, because isolated in his capital he cant really do anything.

Maybe politics like in Cryptic Comet's Solium Infernum is something you are looking for? Have you tried the game? I would highly recommend it! There even the loser can win the match it self, because the politics follow set of extremely complicated steps and rules and fighting seems secondary to the politics - thou politics can be enforced with war and vice versa - but fighting over all seems less important in king making.