The Heroic Ability Legacy unlock doesn't allow you to pick any ability you like; that would indeed lead to some balancing nightmares.
Now if you introduce a system (i know very little about it so if people got info im curious) that you could choose a skill from another toon. Like a smuggler with a force shield from sage or the dash of the assassin. Or a Jugg with a grapple line or with maul of the assassin doing heavy backstab...
I just feel that some combination will be really OP. I just fear that since the balance is already REALLY FRAGILE on this game, putting such a big system in that require even more balance will be a total mess.
Like i said , i love the idea but that just scare me to death.
There was very careful consideration over which abilities to allow as unlocks. With the limitations of the ability only being usable during your Heroic Moment (requiring an active Companion) and the cooldown on that, you shouldn't see too much 'Heroic Ability spam'. We see the unlock as a fun, limited ability that many players will enjoy. It wasn't designed to unbalance the game.