Guessing that the "free play" clause will not surprisingly involve forests or green cards. The last time they did a handful of freebies not color-restricted was traps, which at least trigger on very narrow circumstances. Could theoretically involve saccing creatures (esp brood tokens) but that seems to work at crossed purposes with the set. (already saccing brood tokens for mana, esp eldrazi.)
There is nothing in life that is free my friend the will not give you 0 cmc 8/8 with trample. They will however make you sac a few forests or tap out something to pay for the 8/8, but nothing will be free
A little something you might open in your ROE Booster: An 8/8 green trampler that you can play for free.
http://twitter.com/maro254/statuses/10898814171
Quickly followed by:
As is normal for my teasers, everything I say is true but I might not be telling you everything.
http://twitter.com/maro254/status/10898847088
If I were to design such a card, it would probably involve sacrificing a certain number of forests or something like that.
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Creature - Mammoth King
You may sacrifice all your lands and discard your hand instead of paying Gulhom's casting cost.
Trample
8/8
That's too strong in older formats, with madness, flagstones of trokair, and stuff like that. And Dredge.