Almost There: Red-White Path of Mettle
Figuring out what a format is going to look like after such a big rotation is almost impossible. We're leaving one of the most high-powered Standards in recent memory and narrowing it down to only four sets plus the new one. Overall, any deck you try to make will just look incredibly weak. That said, we already know what guilds will be in Guilds of Ravnica—Selesnya (green-white), Boros (red-white), Dimir (blue-black), Golgari (black-green), and Izzet (blue-red)—so we can brainstorm a little with what we have.
As a disclaimer, I don't actually know any of the contents of Guilds of Ravnica—this is purely conjecture, based on what I think we might see from the guilds we know will be in the set. You'll have to wait until next week to see real previews from Guilds of Ravnica!
One card I've been trying to make work for a while is Path of Mettle. When it was originally released, Path of Mettle made a splash in some red aggro decks, but it quickly fell out of favor to the more consistent mono-red (and later on, black-red). It was always, however, a deck that hovered at the fringe of playability, so I think it can be good if it gets new cards in Guild of Ravnica. Given that Boros is one of the five featured guilds, it's a good bet that it will get something. On top of that, while you certainly lose cards you'd want to play (Hazoret the Fervent and Bomat Courier come to mind), I think you actually lose fewer cards than a lot of other decks. There is one caveat for this deck to work, and it's that we need a good dual land in Guilds of Ravnica. With Guilds being a multicolored set, I think there's a decent chance we'll get one.
Path of Mettle is a card that, at first glance, requires a lot of setup. After all, you need a ton of creatures with those specific abilities. When we look closer, however, we see that a majority of the creatures we want to play in a red-white deck already have those abilities anyway. The reward for transforming it isn't insane like it is with some other legendary lands, but you still get a free land with two abilities that are useful in different game states. If you're being aggressive, a guaranteed 2 damage per turn is pretty good, and if you have to step back and block, then you can punish attackers with the white ability.
I also like that, in red-white, there are two cards that will single-handedly trigger Path of Mettle due to creating two tokens with the abilities you need: History of Benalia and Heroic Reinforcements. Because of those cards, and because some of your creatures have haste, it's very hard for your opponent to run the "cut them off two creatures" strategy; you will often just get to transform Path of Mettle regardless of what they do. Heroic Reinforcements in particular is very hard to stop, since it just automatically flips Path of Mettle the turn it's cast. The two cards are also a good combo, as playing a turn-three History and a turn-four Heroic Reinforcements turns an attack for 2 into an attack for 10.
The biggest decision in the deck is whether you want to play Goblin Chainwhirler or History of Benalia. I don't think the mana can support both (unless we get two great dual lands, in which case all bets are off, but that seems unlikely), so you have to choose. This is obviously metagame dependent, but I think right now people have reacted so much to Goblin Chainwhirler that I'd like to try History of Benalia (plus, you already have Path of Mettle to randomly kill Llanowar Elves and other problematic 1-toughness creatures that show up). This lends itself to a more balanced mana base that can, for example, play Knight of Grace. If the Guilds of Ravnica metagame looks particularly vulnerable to Goblin Chainwhirler, then of course you can revert back.
This is the decklist I came up with pre-rotation:
Obviously, we expect to improve in some fronts. The two- and three-drops, in particular, aren't the power level I'd want them to be. I think you ideally don't play Relentless Raptor or Captain Lannery Storm, for example. Given the aggressive focus of the Boros guild, I think it's almost impossible we don't get a good two- or three-drop to replace them, and hopefully that card will have first strike, vigilance, double strike, or haste.
I'd also like another way of dealing with Goblin Chainwhirler; a 3/3 first striker is very hard to attack through, and with Abrade and Chandra, Torch of Defiance rotating out, we have to play Baffling End if we want more than just the four Lightning Strikes. If we could get any sort of two-mana 3-damage burn spell in the Boros colors (coughLightning Helixcough), that'd be ideal.
Overall, it's hard to say which direction the aggressive red decks will turn. It's possible they stay mono-red, given the commitment needed from Goblin Chainwhirler. However, if they end up two colors, then they're more likely to be Boros than anything else (since Gruul and Rakdos likely won't get a dual, and certainly won't get any cards, and Izzet is not that aggressive), so I think Path of Mettle is a fine place to start.