Mulligans
Aggro Mulligans
Volcanic potion, frost bolt and Primordial Glyph are great tools vs aggro as we already know from existing meta.
Babbling book is always playable, and breath of sindragosa should be a great add-on from this expansion.
Control Mulligans
Early value creation from netherspite is great in a matchup vs control, you also want to get pyros on the board so its less likely to get transformed. Card draw from AI brings you closer to Frost Lich Jaina, and also gives you more tools vs control decks. Frost Lich Jaina is a must keep vs slower control decks.The addition of Sindragosa , Cobalt Scalebane and Bone Drake opens up a possibility to make a dragon mage deck in combination with the existing standard cards. Because of the addition of Frost Lich Jaina for heal and value, i have also included Pyros for some more elementals and Polymorph for transform removal that also syngerises great with Frost Lich Jaina hero power.
Because of the strength of the deathrattles in the deck i see no reason not to add N'Zoth, The Corruptor as an extra win condition.