I'm sorry, but It won't happen. There is no way Putin will leave before he dies for some reason.
'Сonstitution voting' is the best proof of this point.

But if we will accept this magic conditions, there are three ways:
1. Some Putin-like shit from current regime leaders.
2. One of Navalny's friends or Navalny himself.
3. Some stange unknown opposition leader, that wiil appear straight from nowhere right before the elections.