I think that this is perhaps the wrong approach to Advanced Concepts in Feinting. Unlike Blindside, where you've incentivised the Feint behavior by saying "this card gets better when you cast it mid-combat — precisely when you need it most!", this cards says "yo, bro, you can kick this card midcombat to disrupt your aggression, but it's gonna cost you!"

This is the type of design that's going to make players think that Feint works like Ninjutsu, as it's just not intuitive that one would want to drop a 5/5 Trampling Durdle midcombat if it isn't also attacking that turn, especially in G/R. Compare to something like:

Darrowhill Dissident
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Creature - Giant Warrior (C)
Blocked creatures you control have trample.
Feint
3/3

This is definitely a bit too subtle, but it allows the card to impact the board in a greater way when Feinted without behaving at odds with the mechanical function of Feint.

In looking at this an hour after posting it, I actually think the biggest problem here is that it's a common when it should be an uncommon.

I kind of see what you're going at with the ninjutsu comparison, but I think that by having several simple green creatures with feint at common feint, the concept of a creature with feint should be pretty clear. To your point, all of those creatures have feint for the purposes of hitting the board faster than normal. I could see this guy having the same feint cost as his normal cost, but having a simple combat-relevant trick attached as an ETB trigger. (Target creature gets +3/+0 UEOT?)

I think it'd be a great boon if you sketched out a division of mechanical identity for Feint and how that identity might mutate across color, type, rarity and through the progression of the block. At the moment, the use of Feint as a cost-cutting measure (as seen in each color) is successful because it allows the player to sacrifice one form of tempo for another, an equivalent trade that competently portrays the flavor of the keyword.

Though Darrowhill Dissident and Blindside superficially explore the same territory (FEINT DIS TO MAKE BETTER), Blindside succeeds because it plays into the flavor and offers a justifiable and logical trade-off of sacrificing some potential damage for an opportunity to kill a bigger dude. Doing the same thing with a creature gives you three possibilities:

At Feint Costs less than the normal casting cost: Same flavor as other Feint creatures, only now with more upside!

At Feint Costs equal to normal casting cost: Basically same thing as other Feint creatures.

At Feint Costs above normal casting cost: As seen here, you have a kicker-creature with strange timing restrictions and no apparent flavor. ("My Argothian Swine became a Giant Warthog because he was invigorated by the classy way one of his fellow combatants decided not to stab Hitler.")

ETB effects, as you've suggested, offer much more enticing and flavorful designs because you get things like the beginner thief that uses combative distractions to successfully steal:

Mentalist Apprentice
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Creature - Human Rogue
Feint
When ~ enters the battlefield, if you paid its Feint cost, target player puts the top five cards of his or her library into his or her graveyard.
2/3

or the suicide bomber:

Crossblighted Bastard
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Creature - Mutant Soldier
First Strike
Feint
When ~ enters the battlefield, if you paid its Feint cost, you may sacrifice it. If you do, ~ deals 3 damage to each blocking and/or blocked creature.
3/2

So forth and so on.

I see what you're saying, and I think your blue creature design is pretty cool. It feels like an uncommon to me, which makes me think more and more that creatures that do things for the feint cost just naturally fit in that space.

I'd been reluctant to give blue or black more than one or two feint cards as I wanted to keep the ability slanted more to one side of the set, but if I find that those colors want more of the mechanic, I think your Mentalist Apprentice would be a really good piece in the puzzle.

Some other cool concepts that could be explored at higher rarity, or just in the second set, are things like rebel operatives that have a main casting cost in one faction and then a feint cost for an opposing faction.

Faction Insurgent
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Creature - Donkey Rebel [R]
Feint
When ~ enters the battlefield, if you paid its feint cost, put three 1/1 green and white donkey rebel creature tokens onto the battlefield. Otherwise draw two cards and lose 2 life.
2/2

Regardless, it's good for you be reluctant to spread the mechanic into every color. I'd say a great part of Innistrad's success as a limited format is due to the mechanical compartmentalization. It's almost startling how stingy they are with their keywords these days, as Morbid is only on a single non- card, and only 15 cards total.

This last suggestion here is a really cool one, something that I think is a great idea for the third set as the threads of the conspiracy begin to unravel and the true nature and motives of various story players are exposed. I'd really like to revisit this. It's not right for this set, but it's awesome.