Existing Card Reworking - Cockatrice / Elite Cockatrice

[Avenger]

Maybe with a shorter time and temporary HP boost. But then, it is just a new ability to code.
The only reason to have cockatrice do something is that it is 'famous' of its petrifying ability.

[Essence]

Honestly, while I see what you're trying to do, adding Petrify to a Cockatrice defies the entire point.  The Cockatrice is supposed to represent the pinnacle of a 3-casting-cost creature.  There's nothing else that comes close.  If you change it, you have to create something different to fill that slot.

I personally went out of my way to upgrade 6 Cockatrices before I upgraded any other cards (after I had finished by FG farming rainbow and my t50 farming Speed Poison decks).  It's just SOO nice to be able to spam decent hitters that aren't Oty fodder from Turn 1. 

Don't change my hitters, man!

[Kael Hate]

And your reworked Cockatrice is quite strange. Why is Entropy the element to Petrify? Petrify is a very strong ability that shouldn't cause any randomness or dice rollings.

I think it should cost like..  ?

Well I went with Entropy as the period of petrification is random, not the full length of Basilisk Blood. Entropy element also represents forceful Change so it seemed correct there too. Earth didn't seem to be anything special and in anycase where you would have acccess to the Nymph she would be made obsolete by the cockatrice if you were using the Earth element. I'm happy if you want to create the card with Earth activators but I think 4x Entropy is better to both keep the card undercontrol on its own and not overide other cards like the nymph or Arctic Squid.