Gideon, Ally of Zendikar can churn out a 2/2 Knight Ally to bolster your token army. If you don't want to go big, he can turn into a massive indestructible creature. If you already have a huge army and Gideon has 5 loyalty, you definitely want to consider the -4, as it doubles most of the tokens in the swarm.
Nissa, Voice of Zendikar slowly crank out an army, then eventually helps you go big. But, the ultimate isn't as relevant as the first 2 abilities, which create and boost tokens. It's basically a substantially stronger Retreat to Emeria, and can get you out of a pinch in the late-game if you need it.
Sandwurm Convergence fills 2 important roles: creating big tokens and defending the skies. The majority of the tokens in this deck are 1/1's, so it's important to have some big guys to block powerful creatures with trample. Other than Angel of Invention and Westvale Abbey
, there is no flying defense, which is a problem solved by this card.
Hangarback Walker, unfortunately, is still pretty weak in the format (especially when playing against Silkwrap and Statis Snare:), but I still love playing it. There are still plenty of matchups where it will do a ridiculous amount of damage, and he provides some aerial defense upon death, lessening the need for Clip Wings.
Scion Summoner isn't the most exciting Eldrazi, but it's 3 mana for a 3/3, which is never bad. It's very useful when combined with Nissa's -2 and Gideon's emblem, and you can use the Scion to ramp up for a bigger creature, like Angel of Invention.
Secure the Wastes, in my opinion, is still the king of all the token makers. With a single Gideon emblem or Nissa -2, the tokens already become formidable, and if you can use the planewalker abilities more than once, it's insane value. Since it's an instant, you can use it as an ambush block that your opponents didn't see coming, hopefully costing them a powerful creature. But, it's best use is to make little guys to sacrifice to an even bigger threat....
Westvale Abbey
is the sole reason for having so many lands in the deck. This is arguably the best deck for transforming it fast, and your opponents can never see it coming if you pair it with Secure the Wastes. Before it transforms into the 9/7, it can create its own token army, which is awesome when combined with the planeswalkers.
Oath of Nissa is the glue of the deck and helps keep everything together. It finds Westvale Abbey
your planeswalkers to play with, and assists with the occasional mana fixing problem.