For a change it'd be interesting to have a de-aged fic where the de-aged person doesn't return to their actual aged self. Either by choice or circumstance or because that's just the way it is. Could be the other assassin choses to become de-aged as well. Could be it's not Clint or Natasha this happens to, but one of the other Avengers or someone from SHIELD, but Clint and Natasha end up looking after them or featuring in their life, or just plain talking about it.