I'm not exactly sure what it is you want since one minute you're saying that transforming in the open is a stupid idea and therefore you deserve the punishment, then you're saying that it wouldn't do anything to help people getting attacked while transforming.
And both are absolutely true. If you transform out in the open, you're going to get attacked. Hide. Wait until the coast is clear. If you don't, you're going to get attacked until the moment you're able to fly away, and by then you have 3/4 of your HP if you're extremely lucky. Your idea doesn't protect the player. It forces a player's opponents to use more powerful attacks to make up for the inability to strike once using a melee attack.
So what can we do? Well, we can protect the player by making him invulnerable every time the trans starts. But that would be completely unbalanced by making an already superior foe invincible, allowing him to both avoid all damage as well as trans mid-fight, allowing him to use it as a shield whenever the going gets tough.
We've already established your idea does nothing more than push away a silly opponent who would rather melee you once than hit you multiple times with much stronger attacks, rendering it useless.
I can't in good conscience attempt to support an idea that I can't see working, nor can I support an idea that would essentially further nerf the game. Players don't need to be coddled. If you're going to transform, you're doing so at your own risk, but with a great reward in mind. Nothing worth having has ever been easy to obtain. Ascending shouldn't be no different.
By dismissing this idea, I'm trying to save everyone the time and effort you'd waste on a dead-end idea.