I've been playing MTG with an eye on the competitive scene since Theros, and in the recent year I've gotten much better at the game, as well as just using better decks.
My list isn't set in stone, but I copied Gerry Thompson's list from the US Nationals and made some minor changes.
I'm using 4 Bristling Hydra and 3 Glorybringer because I think Hydra is a better curve than the dragon, and also I think the extra energy and hexproof pump are better for fighting U/B control.
I'm using 1 Confiscation Coup main and 1 side, as it is a decent card but I think the situations it is useful in aren't common enough to have 2 main. In its place I have 1 spell pierce (considering changing to supreme will or censor).
2x Appetite for the Unnatural
1x Supreme Will
2x Torrential Gearhulk
1x Chandra's Defeat
2x Nissa, Steward of Elements
1x Magma Spray
1x Confiscation Coup
1x Essence Scatter
Sideboard I have 1 chandra's defeat, not 3, as it is a very specialized card.
The 2x Nissa is because she is a very relevant threat in the lategame versus decks like U/W Approach and U/B Control. Slamming Nissa X6 and using her ultimate is incredible. Also in slower matchups the ability to scry 2 every turn and sometimes play a card off the top can be extremely relevant for gaining board/land advantage. Also, the main reason I think Nissa is extremely underrated is that her 0 ability does not CAST the card from the top, it simply moves from the top of your deck to the battlefield. Also, the card is not REVEALED you LOOK at it, meaning that if the 0 ability is not countered then the opponent has lost all interaction until the complete resolution.
I'm considering upping the count of Abrade or Magma Spray because I think many people will play some version of vehicles at the GP. I was watching Mardu Vehicles on MTGO leagues and believed the deck would be really good with people not having artifact hate in their sideboards lately. Ben Rinauto showed us that with his Top 8 showing at the US Nationals.
Spell pierce is also a card I think many people are forgetting about, as it is incredible versus tokens/approach lists, but negate is often better post board.
I've been playing a lot with two of my friends who are also going to GP Atlanta, which is where I've gotten most of my testing done.