Personally, I would have preferred for skills at certain legendary levels to provide more tangible benefits for the related skill.
Like, say, increased overall damage for one-handed skills at certain legendary levels (with higher legendary levels allowing for higher overall damage).
Higher damage? Never thought o'that. Wish Bethesda had yer imagination...
I am simply using that as an example.
I could definitely think about something more impressive like faster attack speed for melee-oriented skills (going faster for each legendary level).
Other examples include higher chance of success for pickpocket, bonus health/stamina recovery for restoration, higher max charges/stronger enchantments overall for enchanting, damage reduction (physical or magic) for any armor skill.
"Ambition and greed rule, while dogs and swine serve"-Tactics Ogre, Ogre Battle Saga Episode 7, Chapter 3, Chaos route
People would just grind up to legendary 11 smithing anyway. Then, proceed to complain that at level 131 with a sword dealing paralyze with ever shot and a few thousand damage the new legendary difficulty is too easy.
Either way, I think we don't need any new gimmick to sell the legendary thing. Even adding +15 to your skill each Legendary level is too much. There's simply no need for it.
Remember: IamI3rian told ya. Enemy MCC's shields at 50%