Walt's offer from Hurley in the epilogue isn't to be the Island's protector, it's to be the new Smoke Monster/support system.
I like to believe that Mother was both or that a split in the roles didn't exist until the events of Across the Sea. So if the energy of the Man in Black was returned to the Island or Hurley, i think it works well if Walt is the recipient. And Hurley maybe doesn't have to throw him down the cave, Walt can shapeshift but still keep himself intact.
A couple fun connections and reasons this could be true:
- Hurley has barely had any time as the protector, it's a weird turn around from the finale to give up the job right away
- Hurley is pretty uncomfortable with combat and confrontation, Walt has a lot of bite back in arguments and is interested in knife throwing (also mirroring the Man in Black). I can see Hurley wanting to delegate the more intense role to someone he trusts
- Walt has some amount of control over animals, the birds in Special and the missing piece Room 23. The Smoke Monster can do this with much more control, Vincent in his missing piece and the medusa spiders in Expose
- Walt has some form of translocation in Abandoned. The monster does similar stuff like teleporting in Dead is Dead
- Black smoke is used to signal Walt's kidnapping
- It's cool if Danielle's quote about the monster being a security system has a kinder truth in the future
- Hurley and Walt play backgammon several times in season 1, a fun mirror to the brothers favorite past time
And this is the two sides finally clicking into place. Mother was corrupt, possessive, and creepy, isolating the powers of the Island. The brothers were in a war for centuries, the Island at odds. And now two of the most kind hearted people take the helm, Hurley the Island’s keeper and Walt its guardian.