Here's the idea:
At a certain legacy level, you unlock the option to have one of your family members (other characters) join you as a companion. This could scale to level.
For example, I'm playing with my L40 Shadow, and I can summon my L50 Vanguard to be my companion. He keeps his gear and everything else, but some kind of scaling is done to make him proportionate to a L40 base.
It would be fun in many ways.