Spoiled on Twitch
Creature - Elf Druid
Whenever a land enters the battlefield under your control, proliferate. (You choose any number of permanents and/or players with counters on them, then give each another counter of a kind already there.)
I don't think this is great, but it does have the potential with Fetches/Fastbond/other land shenanigans to put a walker to ultimate on the same turn you play it. Needless to say that is good though with the new Nicol Bolas you can literally +4 him, which is only 2 fetches in one turn, and literally ultimate win the game on the spot against some players.
As far as green walkers go to combo with this, Kiora, the Crashing Wave seems pretty decent a game ender.