There are a LOT of posts on this thread, so these ideas may already have been mentioned, however it cant hurt to give it a go.
1. Career mode
2. Team Moral should be included, if the team is beginning to get demolished naturally there moral will be low, this will impact the game, either more knock ons, more turnovers, more failed lineouts, more failed passes (too far ahead, too low, too far behind). (high moral would improve performance). Low moral could be combated by bringing on an impact player to slightly increase moral and try get the team back into the game.
3. Players should have a chance to mess things up while playing. i.e. If a prop tries to take a drop goal or clear a ball with a kick, he would be noticeably crap, especially compared to a fly half. You could implement a random chance that the kick would go wrong and would either go completely off track or not far enough or something. This could be implemented on all players meaning even a fly half could mess up a kick, although it would be a lot harder and rarer for someone playing the correct position to mess up. (this feature could be linked with moral). It would add some realism to the game in the sense that no one plays the same every game, you have good days and bad days.