An assassin that switches places with an enemy to get to the backline, or even to turn on the person they switched with. The cast paradigm for the skill could be similar to Illaoi's Test of Spirit (a colliding skill shot that does nothing unless it hits a champion) or Zed's Death Mark (a relatively slow targeted dash that gives the target an opportunity to reposition themselves).
An bodysnatcher that Freaky Fridays the enemy assassin in order to assassinate their own backline, or anyone on their team to achieve any purpose. This could exclude ultimate abilities for more simple balancing. Following on from above, except instead of trading positions the two summoners trade champions for a short period of time.