When you get to level 50 legacy, I think a good reward is to unlock all datacrons you've gathered on all your characters.
For example, my trooper has all the datacrons and when I get to level 50 legacy, all my toons should get these datacrons unlocked on them too. Except of course for the republic only datacrons, the imperial characters would have to gather the imperial datacrons themselves. It beats having to get all the datacrons on all characters, getting them the first time is annoying/boring enough.