Mulligans
Aggro Mulligans
Only keep Power Word: Shield with Radiant Elemental.
Prince Valanar is a 4 drop, but its life steal and taunt could give a lot of value against a aggro deck that can't deal properly with it.
Happy Ghoul and Spirit Lash are top picks.
Midrange Mulligans
Follows the same logic as aggro on most picks, depending on matchup. In some matchups you might want to not keep Valanar.
Control Mulligans
Radiant + PWS is usually a great start. Keeping Netherspite and Elise can be good, depending of the control matchup.
A midrange dragon deck adjusted with new cards of the new expansion.
Shadow Visions can be combed with Elise the Trailblazer for a late-game card boost, or be used to find that Dragonfire Potion or Shadow Word: Death when you are in dire need for it.
Drakonid Operative is a super strong value and tempo play, and Netherspite Historian gives you discover versatility depending on the matchup.
Two new dragons from KFT, Cobalt Scalebane and Bone Drake are added for more dragon synergy, and also because of the individual strength of the cards.
Spirit Lash and Happy Ghoul should help a lot early game vs ago.
Prince Valanar should work fantastic in this deck, because of the great anti aggro life steal and taunt stats.
Considering the value created from the potential multiple Un'Goro Pack, Drakonid Operative and Bone Drake this deck should be able to go a long way against value decks, while still keeping up the tempo the entire way.
Potential changes could be Twilight Drake instead of Prince Valanar .
Potion of Madness instead of Spirit Lash .