Transforming heroes in-battle
Although full hero transformation in Battle has not yet been implemented. It is possible to sort of fake it.
This possible through having an enemy that attacks other enemies (this means all it's attacks will only be able to attack it's allies). The game TimeStream Saga (the old one) does this occasionally in random enemy battles.
If you can have a hero use an attack that instantly kills the hero and kills an invisible enemy (a non-attacking non-targetable enemy that has no picture) and have said enemy spawn a hero lookalike to fight alongside your normal heroes, you therefore have an in battle hero transformation.