I was thinking the other day about players that started at Michigan, and, for whatever reason, didn't play 4 years here.
The two that immediately came to mind were David Bowens and Antonio Bass. I remember thinking when Bowens arrived in the mid-90s he reminded me of the speedy Florida State D linemen that we had so much trouble with in Moeller's first season. In his freshman season against Miami (NTM) my recollection was that he was absolutely dominant. I forget what led to his transfer (I believe academics), but would have loved for him to stick around.
I don't recall Antonio Bass playing a lot as a freshman, but do remember some crucial runs subbing in for Henne while operating out of the shotgun. It seemed like he was good for an almost automatic 5-10 yards in those situations. I assume that with Henne in place he would have played receiver but for his knee injury, but it felt like he also could have been a dual threat QB pre-Denard.
Finally, and this doesn't quite qualify since it involves a coach rather than a player, but I've long wondered how Michigan's football fortunes in the 1950s would have been different if Fritz Crisler had spent another 5-10 years on the sidelines (he was only 48 or 49 when he retired). His last team won the national championship, and in his last five years he had 2 Big Ten champions and 3 2nd place finishes. His tenure always seemed too short to me.