Mulligans
This deck attempts to combine Secret, Elemental, and Death Knight synergies. To be effective, added cards must be independently useful and powerfully synergistic.
Normal Preservation Tools
Frostbolt, Primordial Glyph, Arcane Intellect, Fireball, Blizzard, Meteor
I included a Blizzard for AOE removal rather than a Flamestrike to keep the list from being too top-heavy, however this will depend on average minion health in the meta.
Secret Synergy
Arcanologist, Medivh's Valet, Counterspell, Ice Block, Potion of Polymorph
I believe this is one of the weaker secrets, and although it is only decent before Frost Lich Jaina is played, it becomes a tempo machine (5 mana destroy any minion and summon a board-control-hero-healing-monster)
Elemental Synergy
Pyros , Water Elemental , Blazecaller
Frost Lich Jaina provides a substantive amount of elemental power by itself. These three cards are a light-weight package to supplement the DK power. They are functional before the Battlecry, and insane afterwards. If there were more Elementals in the deck, I would replace Water Elemental with Steam Surger.
Death Knight Synergy
The goal is simple: create as many 1-Health targets to blast with the DK Hero Power.
I have chosen Sindragosa over Alexstrasza because I believe the Elemental Lifesteal created by its tokens and our other cards can reliably replace the 14 health heal/end turn play after a popped Ice Block. If this deck fails to adequately recover after Ice Block, then I would swap the cards. If Sindragosa is not too slow, its value creates a win condition. The Curator increases the consistency of this combo, and may also draw Stampeding Kodo which I predict will be a strong counter to several new additions to the meta (Voodoo Hexxer, for example.) If The Curator does not draw enough to pay for itself, I would replace it for Flamestrike.
Polymorph is strong without Frost Lich Jaina, and insane with it. As mentioned, Potion of Polymorph is decent but I think its new synergy will make it valuable.