DedWards

Nice catch on Shadows of the Past. I didn't think of using the deaths beyond Falkenrath Noble. I would use Blood Artist, but Heartless Summoning kills it) and Pawn of Ulamog (for more creatures that die).

I like the 'All-In' route you've taken. I only have one suggestion: Elusive Spellfist

If you do stick with Modern, don't forget Transmute. Heartless Summoning and Myr Retriever cost 2, which allows for 3 cards with Transmute: Dimir Infiltrator; Muddle the Mixture; and Shred Memory (all have uses other than Transmute too). I'm looking to aquire a set of Beseech the Queen for the build I'm working on. Also some creature retrieveal (ala Disentomb), maybe Corpse Churn as it's an instant, to 'protect' the combo from graveyard hate. Needless to say, I'm trying for mono black.

Krark-Clan Ironworks is another way to loop the Retrievers.

I also suggest a form of search, as the combo requires 4 pieces, two of which are the same card (Myr Retriever).

You're not running Snappy, so Dryad Militant is an option. It hates out Snappy and Storm that relies on Past in Flames and Pyromancer Ascension. Also, it's a 2/1 for one mana.

Spreading Seas is worth considering too. It may not be a creature, but it hates out lands, like Eye of Ugin (dam those Eldrazi).

Hardened Scales can help put even more +1/+1 counters on creatures.

Fate Transfer is another way to move counters. As is Simic Guildmage. And with Hardened Scales out, the move actually increases the number of counters.

Solidarity of Heroes doubles the counters on target creature. If you target a Heroic creature with this, the Heroic trigger will go on the stack above the spell, allowing it to put an additional counter on before the spell doubles them. Solidarity can even be put on Isochron Scepter, as can Bioshift and Fate Transfer.

Canopy Vista can also be fetched with the God

Esper doesn't sound like a bad plan. It's got access to some of the best kill and control cards in the format.

Sigarda's ability says; "Spells and abilities your opponents control can't cause you to sacrifice permanents."

So, yes, it prevents the negative part of Annihilator, that's not ab ability that features in the Aggro Eldrazi decks being played.

Also, how does it prevent Fetch Lands? It prevents your opponent's from making YOU sacrifice, not them sacrificing their own, nor you sacrificing your own.

I don't have any problem swapping out Scatter the Seeds for something, but Sprout Swarm stays, it's just too powerful. More often than not, it provides more Saprolings than the Thallids (or just as much) per turn.

Also, a set of Evolving Wilds wouldn't hurt.

I really like the idea. I've actually been thinking of making a similar deck and have a few suggestions (I'll try keep them budget friendly).

Firstly, morphing a creature costs {3}, so you won't be doing much in the first two turns and only flickering from turn 4 onwards. To remedy this, I suggest adding some stuff that generates mana. If you where running green (like I will be), I'd suggest adding a set or two of mana dorks. But you're not running green, so I'm looking at artifacts. Azorius Signet seems like the best fit.

Next, I feel 14 creatures is too few. 16 would be a good number, and I have a few suggestions for the other 2:
Vesuvan Shapeshifter - copies any creature on the battlefield.
Whip-Spine Drake - morphes for {2}{W} and has flying. Not impressive to flicker, but a 3/3 flyer is pretty good.
Ponyback Brigade - Enters the battlefield with three 1/1 goblins. Quantity over quality here.

Bloodthrone Vampire might be better than Devouring Swarm. It costs one black less and gets +2/+2 per creature sacrificed, but doesn't have flying.

Rendclaw Trowis another possible persist creature.

Red instead of white opens up not only burn, but cards like Impact Tremors and Fling.

Besides Path to Exile and Selesnya Charm, Surgical Extraction from sideboard can help too. Just target the titan in the graveyard while its shuffle back effect is on the stack. Sure, he still gets to shuffle everything else back, but the titan, and any other copies of it, will be exiled, preventing further shuffle-backs.

Exactly. I looked at the lists being played, noticed a trend (lots of creatures) and build with that trend in mind. If there's a problem, equip yourself with tools to solve that problem, in this case, include more kill.

Eyup. It's a key reason I added it to my Midnight Gond deck.

Have you considered Sprout Swarm? Is horribly op in a Thallid deck.

Playing landfall, you want the lands you search for to enter the battlefield asap. So there's a lot of sub par land search in here.

Map the Wastes; Seek the Horizon; and Sylvan Scrying put the lands into your hand. This doesn't allow for full use of your landfall abilities. Ainok Guide and Loam Larva are even worse as they put the land on top of your library, messing around with your draws.

To remedy this, I suggest taking Explosive Vegetation up to 4x and include a full set of Nissa's Pilgrimage. I've also found that Pulse of Murasa works wonders with fetch lands (Wooded Foothills, Evolving Wilds, etc).

Also, a set of one of the mana dork will help a lot. I use Rattleclaw Mystic; but Whisperer of the Wilds and Leaf Gilder fit the bill too.

One of our younger members (in my club) plays a goblin deck in Standard. He got a few games off me by targeting Zada, Hedron Grinder with things like Titan's Strength (I didn't have enough blockers).

Ooo, Boros. I like me a decent Boros deck. Maybe I should design a Modern one?