Polukranos, World Eater enters the battlefield with four +1/+1 counters
As long as there is a +1/+1 counter on Polukranos, World Eater it has trample and indestructible
Tap; Exile target noncreature permanent
Tap; Exile all other permanents activate this ability only if there are ten or more +1/+1 counters on Polukranos, World Eater
Whenever Polukranos, World Eater exiles a permanent put a +1/+1 on it
Any opponent may tap a creature to remove a +1/+1 counter from Polukranos, World Eater
Just leave that to a mod. No need to backseat mod for something that only adds a little temporary disorganization. I can't see rules mongering from a us normal folk (read non-mods) being helpful to anyone, plus it tends to leach the fun from speculative threads.
I assume the OP is just trying to start a speculation thread about what the hydra does based on the card's rarity, name (which is an important factor when you take into account that Theros, like Innistrad, is designed top-down), and whatever other traits were included on the partial card spoiler from Comic-Con. He could have, however, actually stated the purpose of what he was getting at before his design idea.
That said, I personally think that the propsed design idea, while decently fitting flavor wise, is likely a bit too strong since it's efficient repeated removal of many permanent types in G, which is far from common. I could see it as an ETB Acidic Slime-like ability or maybe, with the phrase "World Eater" in the name, something that chews only on lands, but more efficiently than slime.
Anyway, that's my 2 cents.
Edit - Features included on Comic-Con spoiler that are relevant are: Legendary Creature - Hydra and 4cmc, 2GG.
we better be speculating... i can not decide if that is strong or not... but it feels strong
lol really? i honestly could care less if my opponents remove the counters because i get to tap every turn but the first to exile anything but a creature............................................ unless he meant that can only be done if he has counters on him