Ephemerate

Casting the card again due to rebound's delayed triggered ability is optional. If you choose not to cast the card, or if you can't (perhaps because there are no legal targets available), the card will stay exiled. You won't get another chance to cast it on a future turn. If you do cast the card, it's put into its owner's graveyard as normal once it resolves.

If a spell with rebound that you cast from your hand is countered or doesn't resolve (most likely because its targets have become illegal), none of its effects will happen, including rebound. The spell will be put into its owner's graveyard and you won't get to cast it again on your next turn.

If a token is exiled this way, it will cease to exist and won't return to the battlefield.

Once the exiled creature returns, it's considered a new object with no relation to the object that it was. Auras attached to the exiled creature will be put into their owners' graveyards. Equipment attached to the exiled creature will become unattached and remain on the battlefield. Any counters on the exiled creature will cease to exist.