At 4 colorless mana, the new Karn can be played in basically any deck, but who actually wants to play it?
Let’s take a look at the abilities:
+1: Reveal the top two cards of your library. An opponent chooses one of them. Put that card into your hand and exile the other with a silver counter on it.
This ability is worse than drawing a card because you get the worst of two options. For this ability to be good, you’re ideally able to use your lands in the late game, or at least you need to have a use for your fifth and sixth land. If any land after the fourth is dead, then it becomes way too easy for your opponent to make this ability worth too little. This makes me think control and midrange decks more than aggressive decks, which is already in tune with what the card is trying to do.
In the long run, this ability actually becomes better than draw a card if you’re looking for something specific. If you draw a card three times, you dig three cards deep. If you +1 Karn twice and then -1 Karn, you can see four cards. This is, of course, contingent on Karn (and you) actually surviving that long, but if you’re under no pressure then this will dig deeper.
−1: Put a card you own with a silver counter on it from exile into your hand.
This is your way to “tutor” for something that was exiled. This is usually going to be better than drawing a card because you’re choosing which card you get. The downside, of course, is that you need to have exiled the cards first. The upside is that Karn doesn’t care how the silver counter got there, so you can grab cards that were exiled from a previous Karn.
−2: Create a 0/0 colorless Construct artifact creature token with “This creature gets +1/+1 for each artifact you control.”
This is the weirdest ability, because it seems so disconnected from the other two.
There are two ways to use this ability. The first way is to just play it in a deck full of artifacts, and start cashing in on Constructs immediately. If this is your goal with Karn, then you don’t really care about the other two abilities—you’ll use them, of course, but you don’t need to maximize them. You could play Karn in Affinity or in the sideboard of Lantern. In my opinion, 4 mana is a little too much for this effect in Modern, but I should probably try it before dismissing it outright.
The second way is to have some incidental artifacts and to use this as a kill condition. There are a lot of random artifacts in Standard right now, especially Treasure tokens, and Karn can make use of them. Alternatively, you can also chain Karns. If you have one artifact in play (say, a Renegade Map), you can play a Karn and minus to make a 2/2. Then, on turn 5, you make a 3/3, play another Karn, and make a 4/4, which leaves you with three 4/4s. On turn 6, you can upgrade it into four 5/5s. Not too bad!
Whether you want to use Karn or not depends on whether you have access to better ways to draw cards. You can probably find better card drawing in blue, but how about in black-green? Or in mono-white? Karn gives a recurring card advantage engine for decks that traditionally haven’t had it. Between Karn, Arch of Orazca, and Treasure Map, you can now have a White-Red Approach deck go toe-to-toe against a blue deck when it comes to card drawing, and you get a bonus awesome synergy to boot (since Treasure Map makes three artifacts, which means Karn tokens start at 4/4).
In Standard, one deck is a perfect fit for Karn, Scion of Urza: B/W Tokens. It’s on the control side of the spectrum, but doesn’t have much card drawing, so Karn’s +1 and -1 will be welcome. It also has a lot of token synergies already, and incidental artifacts—Hidden Stockpile makes artifacts, so Karn can serve as a backup kill condition. Karn plus Anointed Procession is also a powerful combo, and those two cards alone can produce four 4/4s even if you have no other artifacts.
Normally, I like the green splash in this deck. Now that you have Karn, it’s no longer necessary. You also get some number of Isolated Chapel and Vicious Offering, which serves as early-game removal or a late-game way of dealing with bigger creatures, including Glorybringer and Hazoret.
This is what I’d try: