AI
Outside of the game you could set your tactics more specifically with the AI, either for a team or an individual player a la Football Manager. This would increase the strategic aspect of the game in terms of whether you want to play with width, keep it tight, slow their ball down with counter rucking or gang tackles, have your defence spread quickly, which would really add to the depth of the game.
For instance, defensively a slider or setting so that when you make a tackle your nearest player will contest the ball by jackling (to slow the game down) or they will leave the ruck and fan out (to prevent an overlap off of quick ball).
In attack you can set individuals to make certain runs (forwards off of 10 or short crash ball lines etc. or for the winger to stay in the 5m channel to stretch the game).
These shouldn't be limited to one option either, the better the players you have the more often these lines would be run from open play or effective they would be against the opposition, therefore having a more developed AI system that doesn't entirely rely on you to do everything (like in FIFA better players losing their markers, making more/better off the ball runs etc.)
GENERAL POINTS
the passing was dissapointing in Rugby 08, miss passes (even from average players) should be flat and quick not 'looped' as some have suggested, that would make a big difference to the game play and aesthetic of the game...
Once you get to the try line and there is a breakdown you shouldn't be able to smash people backwards from ridiculous angles, this was annoying when you sent your No. 8 running at their 10 5 yards out and he got hammered, again maybe an out of game setting for defensive play in certain areas of the field to increase gang tackling.
SET PIECE
The no. of lineout options in Rugby 06 was far better than 08, more choice like that...
Also with the scrums bit, perhaps u could use the right stick to 'time the hit'? so the game goes into side view and you hear the ref's 'crouch, touch...' and you have to move the R stick forward on the engage? timing could be made really crucial (so essentially a skill that was worth mastering) and would add a new dimension to the scrummaging, with early engagements, one side getting the upper hand, then being able to get a drive on, win penalties etc...
im sure i'll think of some more