Nahiri is pissed off and wins the art battle between her and Sorin, Grim Nemesis. But the war for playability may still go to Sorin because at least people understand what his abilities are good for. Nahiri, the Harbinger has a lot of good text, but none of it feels cohesive or points toward a clear shell to put her in. All of her abilities seem fine in a vacuum and some would be amazing in other Standard formats, but right now she looks underwhelming.
Is she a madness enabler? That works with only a few spells I’ve seen, as there are cheaper and better madness enablers for creature decks. She can protect herself, but only in a very specific way that’s unfortunately irrelevant when Gideon, Ally of Zendikar, Archangel Avacyn, and Chandra, Flamecaller look to be some of the best threats in the format. Her ultimate looks tailor-made to abuse with a card like Hero of Bladehold or Oxid Ridge in a fair setting and Emrakul, the Aeons Torn in an unfair one.
I don’t think she’s a good fit for madness-as-a-central-theme in an RW deck, but if enough decent playable cards like Fiery Temper are legal, I can see getting some bonuses out of it. Otherwise, looting, or rather rummaging in the case of discard before draw, has been proven to be quite a respectable ability—and the +2 boosts her loyalty out of attack range quickly. It’s quite possible that without the high-impact creatures from Khans of Tarkir, planeswalkers may actually live for more than a turn in the new format.
As for protecting herself, she’s not great at it, but what she does seem great at is protecting or avenging other planeswalkers. Play a Gideon, make an Ally to force the opponent to commit some creatures to an attack, play Nahiri and clean up the biggest threat on the table. Jam Chandra, Flamecaller after for good measure and you can just play a Superfriends deck without even needing to touch another color.
Her ultimate, on the other hand, may as well not exist in the average deck. Woohoo—you grabbed an Archangel Avacyn to attack with! Still, that doesn’t mean it’s never useful. Finding a hasty Goblin Dark-Dwellers and getting to recast it the next turn can certainly be a nice value engine.
By not focusing a theme on Nahiri, you remove the pressure for her to carry the deck. She’s better suited to a support role where she helps find and protect Gideon and Chandra, much to the chagrin of people following the storyline. Fiery Temper currently feels playable on its own and the bonus with Nahiri is exactly that. Lightning Axe also seems quite good as a 1-mana instant speed answer in a format where Avacyn and Goldnight Castigator are common.
The biggest drawback is the lack of card advantage compared to its Mardu counterparts. Going all the way back to Boat Brew, the Boros decks typically had some other form of CA, whether it be Ranger of Eos digging up Figure of Destiny or Reveillark. This build relies entirely on the planeswalkers to do the heavy lifting in that department and it may not be enough to justify it. Closing a game out should be easy though between Angels, Chandra, and Secure into Westvale Abbey.
Of course, you don’t need to limit yourself to Nahiri’s colors—in fact, you can break all the rules and do a tag team!
The addition of Painful Truths and Sorin, Grim Nemesis locks in a never-ending supply of gas and you still have room for the sweet Secure–Abbey gotcha. Your removal also gets a boost with Ultimate Price, which is looking more and more relevant as the rest of Shadows over Innistrad gets spoiled. You also get the traditional bonus of blanking much of your opponent’s removal or limiting it to Ally Knights and 1/1 Soldiers.
Realistically, this type of deck may end up too slow to see play or you may cut a card Nahiri for more removal or life gain. I want to play with her more before dismissing her outright because unlike Narset, she can actually do something to affect the board. I’m just worried it won’t be enough to justify right now. I’ll see what remaining madness cards get spoiled and revisit her. It would be a tad disappointing for 3 of the 4 planeswalkers to end up playable but not Nahiri.