This will depend on how the legendary powers are made. We do know that they won’t go the route of increasing a skill’s damage by hundreds of percentages. They are there to add further customization options to your character.
There is a good chance that you might not want a legendary in every slot or have legendary power in every slot. This is how it is looking at the moment and I hope it stays going down that direction.
Where this way your items will be a source of your character’s power like it is in all games like this one.