Mulligans
Aggro Mulligans
Keep Netherspite only if you already have a dragon, or it's just going to be a dead card in your hand.
You can keep twilight drake if you have netherspite or twilight welp, trades well against aggro.
Without dragons, replace netherspite and twilight for priest of the feast or glimmeroot.
Control Mulligans
You want to snowball early with power shield and kabal talonpriest and draw by healing your big health minions.
You can keep drakonid op in very control matchups if you already have a dragon or netherspite.
You can keep raza if you already have a great curve, otherwise mulligan for better early game;
Wild Dragon RenoPriest
Gagdetzan Dragon RenoPriest with Lyra/Radiant Elemental and Elise the Trailblazer/Shadow Visions