Change 2h animations so they're like 1h, in that each swing is specialized. Having so few disadvantages to any of the 2h animations, I feel that the thrust is the only one that is actually specialized. If anything, I'd say change the horizontal swings so that they're more specialized, having clear disadvantages like the 1h ones do.
Thrusts are the only swings that 2hers have that really have any downsides. I don't believe that the thrust by itself is OP. I believe however, that the combination of their animations, their swing damage, and their speed, that allow two-handers to utilize the thrust so effectively without any downsides, makes for an overpowered weapon.
Disclaimer: By 2h I obviously mean greatswords, and not just the ones that have "Greatsword" in the name.