There are some flashy cards soaking up a lot of attention in Oath of the Gatewatch, but there are some subtle elements as well. Expedite is a card that is getting some attention as a card that could have significant impact on Standard, and Slip Through Space gives us another one mana cantrip. These cards play very well with the prowess mechanic, and one of the most interesting ideas that I've heard is to play them in a Jeskai Ascendancy deck- the biggest "prowess" effect available in Standard. An idea that a local player was working when I was at FNM last week was to use Ascendancy and these cheap cantrips to kill people in one big turn by using Rattleclaw Mystic and Elemental Uprising to generate mana. Here's an early draft of the deck:

This deck doesn't do anything. And then it kills you. It's perfect.

It's probable that some form of disruption is necessary in the form of cheap spot removal or Dispel to back up your combo, but at least until the deck is a known quantity you should be able to steel wins in game one. Until it becomes clear that you need maindeck disruption, I like just running this completely streamlined list with maximum ability to filter through the deck and find the combo pieces. You want your odds of untapping with Jeskai Ascendancy and be able to just kill the opponent to be as high as possible. Your opponent will probably just kill Rattleclaw Mystic on sight, but at least the grip of draw spells will help you search for backup creatures at a much faster rate than your opponent can find removal spells.

Expedite is great in this deck, as it enables you to go off with Rattleclaw Mystic the turn that you play it, and it also plays very well with Jace, Vryn's Prodigy. If you're concerned that your opponent is going to kill Jace and you have at least three cards in your graveyard, you're able to just Expedite in response to the removal spell which will enable you to activate and flip Jace.

Wandering Fumarole acts as your third option for a creature that "combos" with Ascendancy, which might seem worse than Lumbering Falls, though ultimately I don't think Lumbering Falls is actually workable. Hexproof would be nice, but you're usually going to need to cast multiple red and blue spells on the turn that you go off, so that makes a UG land kind of a non-starter. Also Fumarole just works way better with the manabase given the lands that you can fetch in this color set.

If you love durdling and/or are a combo player, then this is a great starting point for Standard. The deck has some clear vulnerabilities, but you're able to go off on turn four, and you have a lot of ability draw into more action assuming that your opponent doesn't pressure your life total too aggressively.