I think the idea has merit but after having helped my wife level up being by her side as much as possible, I'm not terribly sure this would really be a huge benefit to those leveling. New players are going to have to play through the main story line to level and unlock content. Much of the EXP from the main story line comes from completing quests and that is not diminished by being grouped. Most of the content either requires you to be solo (Duties) or gives such a small EXP that the drain in negligible. Beyond that, the rest of what a new player would encounter is either Dungeons or FATEs. Both of these already scale so the issue with loss of EXP is not an issue there.

Again, I'm not saying it is a bad idea but I would like to know specifically what areas you feel are taking a major EXP hit due to the lack of a mentor system.