Mulligans
Aggro Mulligans
Depending on what sorts of boards your opponent is likely to make, Warpath and Brawl might be good keeps, too.
Combo Mulligans
Depends on the kind of combo deck. Inner fire decks you can stall with removal, so normal board control tools are good. Against another Mecha'thun style OTK deck, all you can do is power cycle and hope you get there first