It would be very fun. Probably only do this one if there's some sort of leadable entity interface

Leashing can be controlled by overriding methods in MobEntity, which swets do extend from (SwetEntity -> SlimeEntity -> MobEntity); however, the actual logic to control the leash moving the entity, breaking, etc. is implemented in PathAwareEntity, a subclass of MobEntity, which slimes and swets do not inherit from. To allow leading swets, the logic would have to be reimplemented.

Coming to a 1.6 release near you!!!