It would be cool if you could select a player to line up with someone at all times so your number one CB would automatically line up against the opponents NO1 WR, even if they lined up in the slot or in the backfield(This would be good if you are playing against an opponent who uses the "Package" option a lot)

Yea, it would also be good if your opponent has one particular receiver who is a beast and who me moves in motion a lot to avoid a particular DB.