Buffing two-handed style would definitely be a good starting point, increased damage is an easy stat to come by and 15% winds up being close to nothing. The increased recovery speed or crit conversion other styles offer both scale way better.
One buff I'd like to see for 2-handers is some kind of accuracy bonus, particularly early game. Missing when you only swing once every 4-5 seconds feels awful, balance aside I would enjoy playing with slow weapons more if they didn't whiff as often.