Seems fine to me. We assume we'll tweak the Comp Rules to allow Auras without a target to resolve. I imagine though that if you did cast this with a target, and the enemy killed it in response, then this would still get countered like any other Aura.

Targeted auras can still fizzle before it enters the battlefield. that part hasn't changed.

Could this be "As you cast, put a 0/0..." and then it's there to target; and if you don't, well, it'll just die off?

I ask, because the text has become truly tiny here.

creating a 0/0 and attaching the aura must be part of the same effect.

maybe omit the reminder text for future living auras.

I guess so. The text on living artifact was just about as bad, after all.

Hmm... the part of this card that forces you to have a target are the words "Enchant Creature". Since most people assume that auras are Enchant Creatures by default, you can play off of player expectation by using this wording:

"When ~ enters the battlefield, it gains Enchant Creature. You may either attach it to target creature, or put a 0/0 white Essence creature token onto the battlefield, and attach this to it."

This way, players can respond to a trigger on the stack, and the card targets... it just does it as an enchantment sitting on the board with an enter the battlefield trigger. The reminder text is a little shorter, and you cut an extra line out of the main body of the text: The words "Enchant Creature". Hmm... getting rid of the target, however, leaves the enchantment on the table. I guess you have to use "As" as opposed to "When". That's unfortunate, but it still works.

enchant only restricts what you can target. being an aura is what forces you to target. living aura works around this. trust me, i've read through the comprehensive rules to come up with the current template for living aura.