DedWards

Correction; Goblin Lore is +4,-4 and Tormenting Voice / Wild Guess is -2,+2 (including the card itself). So neither net you a card.

I really like the Locket of Yesterdays, sadly storm is probably the only place it'd work.

Goblin Lore is interesting. While it digs deeper than Tormenting Voice or Wild Guess, it also throws away a lot more. Additionally, while it may net you a card (+4, -3); where Tormenting Voice / Wild Guess doesn't (-1, +1); it leaves you with a lot less control over what you keep.

Reforge the Soul seems like an odd choice. Miracle can only be used as you draw it, and only if it's your first card drawn in the turn. So, 5 mana to refuel the hand seems a bit expensive.

With Young Pyromancer and Empty the Warrens in the sideboard, 1-2 Goblin Bushwhacker could be a nice sideboard option too. +1/+0 and haste to all those tokens you just stormed out (and the Bushwhacker itself) is awesome.

Although Assault Strobe is awesome at 1 mana, its sorcery speed kind of irks me. Temur Battle Rage, while 1 more mana, is instant speed and grants trample in most cases (heroic triggers resolve before the spell that triggered them). So I'd move it to main board.
Coordinated Assault is awesome. For 1 mana it targets 2 creatures, doubling the number of heroic triggers :p

A few more creatures couldn't hurt. I ran a GW heroic deck for a while in Standard and the worst games for me where the ones that I didn't draw a creature. Balance of creature to targeting spells is the biggest problem here. Too many either way and you risk sitting with problems in most of your games.
Akroan Skyguard is good because it has flying. This one was one of the main reasons for most of my wins.
If you can work a few more duals into the deck, Phalanx Leader would be an awesome addition. Its double white does limit it a lot.
If you're not hell bent on +1/+1 counters, Anax and Cymede is a frigging awesome legend.
Another route is to populate the board with token creatures (which would benefit from Phalanx Leader and Anax and Cymede). Akroan Crusader and Vanguard of Brimaz do this well.
I know heroic is the theme here, but you may also want to consideradding a few Prowess creatures to the mix. Monastery Swiftspear; Seeker of the Way; and Monastery Mentor are probably the best options.

Almost forgot, Setessan Oathsworn is a frigging awesome heroic creature. 3 for a 1/1 sounds week, but it gets really big really fast (two +1/+1 counters per heroic triggers). Also, it is a warrior, so Bramblewood Paragon will give it a +1/+1 counter too.

While some other heroic triggers are cool, the ones that grant +1/+1 counters work best (at least they do for me).
Rancor should be main board, and at 4x. It is arguably one of the best heroic trigger cards in Modern, and it's own bonuses are not to be ignored.
Concentrating on +1/+1 counters makes 4x Solidarity of Heroes in main board a must. The heroic trigger's counters for targeting it with Solidarity of Heroes will be doubled too.
Continuing with the +1/+1 counter love, Hardened Scales is a good option. Who wants to say no to extra +1/+1 counters?
Bramblewood Paragon gives trample to any other creature you control with a +1/+1 counter on it. So even though heroic creatures are mostly not warriors, they still gain a solid bonus (trample).
On the same note, Ainok Bond-Kin gives first strike to any creature you control with a +1/+1 counter on it.
Because Warriors' Lesson can target two creatures for only 1 mana, it should automatically be 4x. Cheap, multi target cards are few.
Mutagenic Growth is yet another solid choice. The surprise +2/+2 is further enhanced by a heroic trigger, making the 2 life spent even less of a problem.

Kiki-Jiki + Zealous Conscripts = infinite Zealous Conscripts with haste.

Restoration Angel let's you flicker Springjack Shepherd and gives you a 3/4 flyer at instant speed.
Springjack Pasture supplies goats every turn that you have leftover, unused mana. It can also turn your goats into an explosive amount of mana, if only for 1 turn.
Trading Post is another good source of goats. With all the life gain from Soul Warden, Soul's Attendant, and Suture Priest, paying 1 life for a goat is nothing.
As all your creatures are white, Honour of the Pure is a good option. And depending on how much you concentrate on the goats, Intangible Virtue is good too.
Growing Ranks looks like it could work well with Springjack Shepherd. It is a permanent which has 2 white mana symbols and it'll duplicate a goat every turn.
Sundering Growth is worth considering for the sideboard.

Excellent landfall trigger enablers:
Search for Tomorrow
Khalni Heart Expedition
Explore
Harrow

Additionally, Golgari Rot Farm could help with landfall triggers as it bounces another land back to your hand. As it enters tapped, I'd only run 2, if any at all.

Two words: Sprout Swarm

If I where to add more Nykthos, Shrine to Nyx; I'd probably only go up to 3x. I've already had more games than I'd like to admit where I didn't have enough devotion to make the Shrine worth while. There's a lot of decks in Modern that run a lot of efficient removal, and without the creatures supplying the devotion, the Shrine only adds colourless, which is bad for this deck.

Search for either the Shrine or Rogue's passage is why I have 3 Sylvan Scrying. 4 is overkill, as it puts the land into my hand, but 2 is too few.

A few cards that may interest you:
Wild Pair
Summoner's Pact
Dramatic Entrance
Khalni Heart Expedition
Explore
Search for Tomorrow
Rogue's Passage
Utopia Sprawl
Vigor (I know it's not a wurm, but its ability is awesome)

Also, Terramorphic Expanse and Evolving Wilds don't quite make sense here.

Nice. Hope you have more wins with it.

Chalice of Life // Chalice of Death is a great win condition.

Jeskai would defeat the object of doing this without the blue that all Splinter Twins use. Going Jeskai would also bring the question of why I'm not using Deceiver Exarch and Pestermite?

Also, Gitaxian Probe is already in the deck.

You're missing the best Saproling production card: Sprout Swarm.
Other cards that may interest you:
Obelisk of Urd (calling Saproling)
Overwhelm
Scatter the Seeds

Sunscorch Regent costs 2 more mana than Managorger Hydra, but its base P/T is higher, and it has flying instead of trample, but it also has the +1/+1 counter ability and also gains you 1 life every time it triggers. Something worth thinking about.

Remember, Foundry Street Denizen only triggers for red creatures entering the battlefield. Even if Foundry Street Denizen's ability did trigger for any creature, Kruin Striker is still better as it gains trample.

Some more cards to consider:
Obsidian Battle-Axe / Lightning Greaves / Swiftfoot Boots - they equip for free / cheap and grant haste (among other things).
Giantbaiting - can make two 4/4 warriors with haste. They interact with all the warrior related stuff quite nicely.

Manamorphose is a good mana fixer, but it doesn't net you mana, though it is a 'free' draw spell (costs 2 mana and gives 2 mana back). You need to decide what's more important, draw or land ramp.
Harrow is a brilliant land search card as it puts the new lands onto the battlefield untapped.
Search for Tomorrow possibly my favourite land search card. Suspend it on turns 1 or 2 to have a 'free' land search that puts the land onto the battlefield untapped two turns later. And you can just hard cast it mid / late game.
Rampant Growth is a classic land search as it costs two and puts it onto the battlefield, though be it tapped.

Another card I forgot: Essence Warden. She gains you life every time any creature enters the battlefield, including those you give your opponent. The Warden does one of two things: gain you substantial life to live until you get your key cards out; OR acts as a lightning rod for kill spells, reducing the number of removal that could have killed the important creatures :P

Nacatl War-Pride and all its tokens are Warriors. Bramblewood Paragon will give them a +1/+1 counter each just for entering the battlefield and she also grants any creature you control with a +1/+1 counter on it trample.
Brighthearth Banneret makes warriors cost {1} less to cast.
Primal Forcemage gives a temporary +3/+3 to any creature you control as they enter the battlefield, and as the War-Pride tokens are temporary, this is awesome.

The biggest down side to Nacatl War-Pride is it requires the opponent to have creatures, the more the better. A way to solve this is to give the opponent creatures and the best cards in green that I can think of are Mercy Killing and Forbidden Orchard.

Looking at your mana base, you need to fix it as it currently only has forests in it.

Another trick with Nacatl War-Pride is to duplicate its tokens. Because of the way its token making ability is worded, any copies of the tokens enter untapped, not attacking, and do not get exiled (though the original tokens still get exiled). This is easily achieved through Parallel Lives and / or Doubling Season. Another card that requires a little more work than the enchantments is Rhys the Redeemed's second ability.

Personally, I feel. Zada is a build-around card, not a card that is just included. It's fun when it works, but unless you build to specifically use it more often than not, it falls into the gimmick category.