Overview:

An update of my RNA Familiar Counters deck, to take WAR cards into account.

While the RNA version was largely built around Biomancer's Familiar, with the addition of Nissa, Who Shakes the World this deck has more of a focus on turning lands into creatures.  As with the previous deck, the a good scenario is to have a Helm of the Host on a Hadana's Climb // Winged Temple of Orazca, but the addition of Merfolk Skydiver and Karn's Bastion encourage more proliferation.

Best-case senario in this deck would be to have 2 Biomancer's Familiars and a Merfolk Skydiver on the board the turn Nissa, Who Shakes the World comes into play.  3 proliferates (at {U}{G} per proliferate) would get Nissa to her ultimate ability.

The worst matchups for this deck are aggro, but it should perform solidly otherwise.

Deeper Dive:

Having many mana sinks and turning land into creatures helps take the sting out of flooding.  Ideally, mana uses on the board would be preferable to casting any card that could be drawn, leaving mana open through the opponent's turns to react to whatever they do.  With that direction, the synergies the deck wants to make use of are:

• Lands as More Than Mana
• Excessive Mana
• +1/+1 Counters

Lands as More Than Mana

The core of the deck should be built around treating land as more than mana.  This could mean using lands with additional abilities (e.g., the Memorial cycle), but that often does little to stabilize a board during a flood, and would generally prefer a different card than a land.  More useful for stabilization and improving board state is the ability to turn land into creatures.

This is fairly expensive to do, and often stronger creatures can be added more cheaply, so that should not be the sole focus of the deck, but should drive decisions for the deck.

Some cards that help treat lands as more than mana in this way include Awakening of Vitu-Ghazi, Waker of the Wilds and Nissa, Who Shakes the World.  Awakening of Vitu-Ghazi is only a good draw the turn it's cast and does nothing to make land drops good later in the game.  Both of the others, however, meet both goals.

Excessive Mana

To build a deck where a land is an ideal play from early to late game, it has to be expected that there will be significantly more mana available than needed to cast whatever cards are drawn in a turn.

The obvious solution for this is to have a variety of reusable mana dumps.  Waker of the Wilds is already a good one, creating or growing land creatures for 3+ CMC.  As +1/+1 counters are considered, a couple additional good additions are Shapers of Nature, both to add more counters and to convert counters into cards, and Merfolk Skydiver, as a mana sink to proliferate counters.

If supplied with reusable mana sinks, it's also potentially worthwhile to add more mana.  Biomancer's Familiars will make the mana sinks cheaper, effectively adding mana, while Wilderness Reclamation will make lands reusable every turn.

+1/+1 Counters

Both methods that convert lands into creatures do so by adding +1/+1 counters on them.  Making further use of these counters can add synergies into the deck.  Mentioned above, Shapers of Nature and Merfolk Skydiver work with +1/+1 counters.  Additionally Simic Ascendancy can be used to add +1/+1 counters and give an alternate win condition.

Play Tips:

Keep the Biomancer's Familiars alive.  Try to keep enough mana open to counter anything that would kill them off, and be willing to consider sacrificing a Shapers of Nature or Waker of the Wilds to keep the Familiars safe.

Remember to set the end step stop on Arena, tapping all lands before any Wilderness Reclamation triggers happen.  And don't spend the mana until all mana is in the pool--spending it before untapping can leave unable to counter anything the opponent plays.  Also, be willing to waste mana in the pool after the last Wilderness Reclamation occurs if it keeps a counterspell or two avalaible.

Don't expect to use Hadana's Climb // Winged Temple of Orazca as a method of getting counters onto creatures.  It's better as a 3-cost land, adding the +1/+1 and flipping it the turn it comes into play.

Card Tips:

Simic Ascendancy is generally a red herring, but can win games if the board stalls.  Don't worry too much about protecting it, but keep a close eye on it--it can turn the game around pretty quickly and be a 1-turn clock in the right circumstances.

Grappling Sundew can help stop some damage, but with some counters on it it can make it nearly impossible for a creature to get through and can discourage a lot of attacking.

Helm of the Host may be able to get into play and equip on the same turn, but with lots of mana coming from Wilderness Reclamation, that doesn't happen until late in the game.  If it can't come into play and be equipped at the same time, it may be worth holding and spending mana elsewhere.

Other Cards:

Some cards feel like they may fit, but didn't make the cut:

Evolution Sage -- Proliferating on land drops helps make them worthwhile throughout the game, but they add at most one proliferate per turn without any additional methods of getting lands onto the board.

Llanowar Scout, Elvish Rejuvenator -- Getting more land onto the board each turn could be nice, but other than very early game, any other card (including a land) would generally be a better draw.

The Mending of Dominaria -- Getting cards back from the graveyard and dumping a lot of land onto the board can be useful, but by the time this is out and active, it's generally late game with better uses for mana.