Mulligans
General Mulligans
You need early game interaction and card advantage. Tracker makes up for the weaknesses of singleton, often finding additional copies of either deathrattle cards or payloads such as N'Zoth, Reno, and Sylvanas. Grimestreet can either reinforce your strategy or explore other vectors, such as (gasp) real removal spells! If you have Rexxar in your opening hand, your gameplan changes from holding out to active trading to ensure you can Rexxar on 5/6/8 to great effect.
Aggro Mulligans
Clear, clear, CLEAR. If you get to the late game in a good position, you'll stay there.
Updated version of http://www.hearthstonetopdecks.com/decks/reno-nzoth-hunter-season-41/
Much of the theory there still applies here.
N'Zoth, The Corruptor, Deathstalker Rexxar, and Reno Jackson are your best cards – Rexxar makes deathrattles with crazy combinations for N’zoth, Reno and Rexxar keep you alive to PLAY N’zoth, and Rexxar is your most reliable board clear (if you’ve been trading appropriately.)