One of the most hyped up cards from Oath of the Gatewatch would have to be Goblin Dark-Dwellers. There’s a good reason for this, as the card is insane.
Dark-Dwellers is a 4/4 menace for 5, which is just OK. If I’m going to get a 4/4 creature, I don’t want to spend 5 mana on it if I’m not getting a game-altering effect. I would, however, be more than happy to spend 2 mana on a 4/4 menace creature, which is much closer to what this card actually is.
Dark-Dwellers gives you a free spell from your graveyard that is likely to be valued at 3 mana. Ruinous Path is a versatile removal spell that scales late in the game, Crackling Doom is an efficient removal spell that usually takes out the most powerful option your opponent has, and Kolaghan’s Command is an excellent value card, especially in conjunction with getting back a Dark-Dwellers.
Even if I’m only getting a Duress, Roast, or Fiery Impulse when I need it, I’m still getting a pretty solid deal by casting Goblin Dark-Dwellers. I like having access to Read the Bones, especially if it lets me play fewer lands, but if you want to “splash” colorless spells in your deck, that’s going to be a bigger challenge.
Thought-Knot Seer is amazing and you saw its power during the Modern Pro Tour this past weekend. Taking your opponent’s best spell and leaving behind a 4/4 creature is so much more than you should ever get from 4 mana. The stretch to produce colorless mana is definitely real in a Mardu deck, as going beyond Caves of Koilos and Battlefield Forge (2 cards I am not interested in playing in most of my Mardu decks) is not pretty. Unknown Shores is not a good Magic card, so needing to play it in Standard stings, but maybe the Seer is worth the cost.
Shambling Vents and Needle Spires are both awesome cards, but losing access to the Nomad Outposts that let you play painlands that can produce colorless may not be worth it. It’s definitely a powerful shell no matter how you end up building it, and Goblin Dark-Dwellers is sure to see lots of Standard play for some time.