There really aren't many critical feats, and now that improved crit doesn't stack with keen, it's almost not worth it (just get a keen weapon or a scabbard of keen edges or a keen edge spell cast for you, gp is easier to come by than feats). I'd suggest as you already have the expertise feat for weapon master picking up improved trip as with a two-handed weapon you've likely got decent strength.
Also, if you've got quintessential fighter, maybe Two-Handed Power strike. If not, getting to great cleave can occasionally be very useful.