You want to build a deck that can harness Ashiok as an engine, and anything else is supplemental.
If you're playing against Dredge, and you have Ashiok main and can land it game 1, it's probably too slow, but it could very well turn the game around. (Note: You can play Ashiok off only a Black Lotus, unlike Dack.)
If you're not playing against Dredge, I think you want to be piloting a deck where -1 Ashiok, targeting yourself has the effect of either drawing cards (Yawmogth's Will, Snapcaster, Ramunap, Life from the Loam, etc) or powering Delve (Treasure Cruise, Dig Through Time, Tasigur). You are gaining resources (and knowledge about the remainder of your deck), and hindering your opponent.
For comparison, Bazaar moves 5 cards (2 from library to hand, and 3 from hand to graveyard). This one moves 4. It's not a land, and you likely won't play it on turn 1, so it's clearly not as powerful on the two axes, but it does give us some insight into it's raw max potential. I figure it's somewhere more than 80% of Bazaar. It has two upsides, pitches to FoW, soaks up damage, and doesn't take up a land slot.
I still debate with my friends how much Treasure Cruise or Dig Through Time should cost to be "fair," but they both just got cheaper by four mana. And we already know Yawgmoth's Will and Snapcaster are both busted.
Ghost Quarter just became Strip Mine, and Ghost Quarter has seen play in Vintage. UG could make this a thing. UB & BUG? Probably not. I don't think this is a real point, but I wouldn't be surprised if someone tried a thing.
Play it, -1, Untap, -1, Delve 7.
If you get 3 uses out of it, you can delve for 12, which is 4 rituals worth of mana for 3 mana. It's spread out over turns, but it's not bad.
There's probably some sort of Bazaar, Thoughtscour, or Tasigur shell for it, too.
I don't think Shops will run new Karn. They haven't stuck with any Karn for quite some time, and very few abuse Null Rod, which Workshop can play. New Karn shines as a wishboard/toolbox enabler, especially with Serum, as @zias noted.
I'd love to see a resurgence of MUD Prison decks a la Stax, but there aren't enough hate pieces to lock someone out like the days of 4 Trinisphere or 4 Chalice of the Voids. The hate pieces that are available you already run, and you can't power a Karn through those via Workshop like the rest of the hate pieces. So, I'm not sure it fits in any current shell.
It is extremely powerful and the +1 is nothing to scoff at. You can go aggro, and you can pick off opponent Moxen. You can even get rid of your own Mana Crypt in a pinch.