All-In Zoo has been a reasonably successful Modern deck, but it's also a deck that hasn't been tuned very much. Perhaps the new versions will look more like Hall of Famer Kenji Tsumura's version which went 5-0 in a Modern League. His deck is streamlined and focused on comboing people out, although it's also capable of winning a "small ball" game.
Kenji, AKA Kogamo, played straight Grixis, cutting all the smaller creatures such as Wild Nacatl. His threats are only Death's Shadow, Kiln Fiend, and Gurmag Angler. With lots of cheap cantrips, Kiln Fiend and Gurmag Angler make sense, especially with Mutagenic Growth to defend Kiln Fiend from Lightning Bolt. Stubborn Denial works well with each of the creatures too, so there's more of a protective element to the deck. With only twelve threats, you kind of need something to ensure one of them sticks.
In order to make good use of Death's Shadow, this deck's manabase is all fetchlands and shocklands. There are no basic lands or Scars of Mirrodin fast lands, which would otherwise be pretty nice in a deck like this. You want to be able to cast Death's Shadow on turn 2 if possible, and this deck is good at doing that. The cantrips find your combo pieces, but also help find your land drops early and make sure you don't flood out in the late game. With only four real lands in the deck, it's clear that Kenji doesn't want to be making many land drops.
His sideboard contains ways to interact with opponents who might be a bit faster. Thoughtseize is the best way to handle things, since not only does it take whatever you want, but it also works well with Death's Shadow. Leyline of the Void handles unfair graveyard decks for minimal investment, while Dismember and Spellskite fight the now-defunct Twin combo. Still, keeping around some Spellskites might be worthwhile for the Burn matchup, but I imagine you just want to try and race them.