Fifa is pretty much perfect because there's not a lot more to put in a football game tbh, especially when you've just got a controller to move your player atm.
I think that rugby games need a revolution in a lot of the mechanics.
Rucks
Scrums
Support play
Passing
Realistic consideration of fatigue
Tactical play/strategy:
~ there is much more to pro rugby than just trying to run, break tackles, make tackles and score tries. You have to break down your opposition.
Movement of players:
~ hb studios they all follow the play like robots
~ sidhe they're all over the place with no organisation to speak of
~ forwards should be ready to carry the ball and hit rucks for the most part on attack
~ backs should be running different lines depending on their stats, team strategy/plays, etc... on attack
Of course things like contact/physics, running animations, general ai will improve over time naturally (not that they've seemed to far since 2006...).