I apologize in advance for the long post as I have several ideas.
Heart of Kiran and
Hissing Quagmire seem pretty bad when you can just turn Land into Creatures with almost every Nissa, while
Buried Ruin and
Ugin, the Ineffable are better in Artifact-heavy Decks. I also wouldn't use
Tainted Wood, especially with your relatively low volume of Swamps (although
Woodland Chasm would add one to the quantity), but your mileage may vary.
The thought behind those creatures are simply to protect the planeswalkers. They produce a lot of different token creatures, but not really with reach or flying. The Thornweald Archer and the Wildborn Preserver are included as low CMC aerial blockers, mainly. Will-o-the Wisp might be a better solution. Ideally I'd have more creatures with vigilance and reach, like the Elder Gargaroth, to be able to attack and still be able to block, but I guess that'd mean higher CMC creatures.
Oh, so Heart and
Quagmire are low-mana-value blockers; okay, that makes some sense. With that said, there are four Planeswalkers that can help with Reach specifically (
Lolth, Spider Queen,
Vivien, Champion of the Wilds,
Vivien, Monsters' Advocate and
Wrenn and Seven), but you're only running one of them. The Viviens also support
Vigilance, and Champion of the Wilds can even grant those abilities to ANY Creature, including the Tokens that your other Walkers make.
Ob Nixilis of the Black Oath can make beefy Fliers, so that's a valid consideration as well.
I recommend
Necroblossom Snarl since you'll have a
Forest or
Swamp in hand at least some of the time, and you can up the odds (for it as well as
Woodland Cemetery and the aforementioned
Tainted Wood) with
Woodland Chasm, which you can get onto the battlefield from Deck with cards like
Farseek or
Nature's Lore, which I believe have been mentioned earlier by others. I also use
Nissa of Shadowed Boughs and
Nissa, Vital Force, and sometimes
Oath of Nissa but that's not as good in two-color Decks.
Talisman of Resilience is
Llanowar Wastes in mana rock form and has justifiably been mentioned by others in this thread; I also recommend that card, and use it myself.