Turbo Serpents + (Explorer)

I loved the classic explorer no rares deck so much, I made my own adjustments with rares/mythics to make it more competitive later in the game. Slip out the Back lacked synergy with Pteramander in the original build, and since I was adding another Pteramander, Slip out the Back was an easy one to cut to make room for my rares. Whelming Wave replaces Slip out the Back. It synergizes with the serpents and is a nice midgame wipe. Similarly, I felt Delver of Secrets in the original build fought the deck as a whole due to wanting to keep an instant on the topdeck, so I swapped him for another Pteramander, a Haughty Djinn, The Magic Mirror, and a Sea-Dasher Octopus, who can protect your fliers from your own bounce and gives a little card boost. I found that with 4 Make Disappear I often ended up with one or two in hand in situations late game where I needed a straight counter, so I swapped two for Sinister Sabotage.

As a side note, there is one change I'd love to make if Khans of Tarkir goes up on Arena. The Ferocious ability would work very well with this deck, and I think Stubborn Denial over Spell Pierce would be excellent. I wasn't in love with Strategic Planning as it is sorcery speed and eliminated your ability to set up a counter and cast at your opponent's end of turn. I've played some games swapping it for Thirst for Discovery and Unsubstantiate, and so far I prefer Unsubstantiate, which helps deal with two cards this deck hates (Sheoldred, the Apocalypse and Supreme Verdict). May try Winds of Rebuke, Blink of an Eye / Into the Roil, Ephara's Dispersal, or Unexplained Disappearance, but for now I think Unsubstantiate is it.

Mulligans: Your ideal opening hand with this build is going to be 2 Island, 1 Spell Pierce, 1 Consider or Otherworldly Gaze, 1 Make Disappear Censor or Jwari Disruption, 1 Curate, and pretty much any 7th card, maybe 1 creature. The idea is turns 1 and 2 you are just countering if possible and otherwise playing your draw/surveil at your opp's end of turn. If you have less than two cantrip/surveil cards in your hand you are mulliganing for sure. The creatures you don't really want more than one of in your opening hand. You just grab them later with the instants. Believe it or not, if you have Consider and/or Otherworldly Gaze in hand too, you can get away with just 1 Island in your opening hand pretty often.