Oh, I like it. "Play green, go be stompy. GO ON! you know you want to!"

Not quite sure what the flavour connection between the mechanic and creature type is, mind.

Well, it's the green community thing, which is the flavour of the police in this set - hence collaborate (which was designed about a week before Battle Cry was previewed!). Imagine a police force all mobilising to catch one particularly tricky thief, all working together... I think that's what I was thinking when I designed this, anyway.

In playtesting this turned out quite tricky to trigger. I wondered if the power needed to go up, but Vitenka thought not. Just put it in a deck with a nicer variety of huge creatures and utility creatures.

Also very nice with vigilance.

I wonder what happens if you control no creatures? Fortunately I think it's impossible to remove this guy's creatureness without also removing his ability.

The only way I can see for that to happen is to Cytoshape it into a Licid, then attach it to an opponent's creature (and if you used your Licid as a DNA specimen, then either turn that into an Aura too or get rid of it). Next turn, the copy effect ends and this will be an Aura with the Licid-granted enchant abilities as well as its printed ability.

Then I believe the game is a draw because the state of attacking with all 0 of your creatures is vacuously satisfied at every moment in time.

Just make it trigger during the declare attackers step. "At the beginning of your declare attackers step, if all your creatures are declared as attackers, ..."

Hahahaha. Of course; I've used the Cytoshape-Licid trick myself. I should know better than to say "It's impossible" :)

Frankly, I think the option of letting the Cytoshape-Licid player draw the game if he wants to is better than fiddling the wording to prevent him :)

I would have assumed that "whenever all creatures you control attack" would only trigger at the declare attackers step, even if you controlled no creatures (and hence never happen if you don't have an attack step, completely coincidentally doing what a non-logician would expect, although that's not actually helpful). But I agree it's at least ambiguous (I don't know, is there any official precedent for one interpretation being favoured by the rules?)

If nothing is not a creature, can it attack?

But all of the creatures you control are attacking. Heck, all of the elephants you control are tap dancing. Because you don't control any creatures that are not attacking, or elephants not tap dancing.