Ptah, although what you propose may be realistic, what kind of fun would it be to play a Roman faction? I can't see anyone wanting to play the Romans since their entire strategy/campaign would be dictated by the Senate...unless they want to risk the displeasure of that faction. Also, with complaints many people seem to have with the AI in other parts of the game, I can just imagine the screaming we'd be hearing in these forums with the AI Senate having even more control over the game than it already has.

I think they hit a good game play balance with the Senate as it is. The Senate nudges the campaign direction rather than forcing it and a employs a carrot/stick approach just strong enough to make the Senate's wishes a serious, but not game-breaking, consideration.

As for re-writing the international diplomacy system, it was for this game and it is far, far better than the system in MTW. Each TW game raises the bar on its game features, so perhaps later TW games will incorporate some of the features you listed.