Or other swords could use a buff...
Because greatswords aren't worse than top tier polearms in any way. Greatswords have their advantages, poleaxes have their advantages.
There is way more useful polearms than 2H weapons.
Range is the king of this game, after comes speed. Balanced weapons are just ok. Slightly buffing longsword, heavy bastard sword, bastard sword, heavy great sword, dadao and especially twohanded sword could result in more diversity. Bastard swords could be buffed by adding one more slash damage and making them one slot.
Or buff 2H axes and clubs...
And no, Katana shouldn't get another buff. Actually it should be crappy compared to 2H euro swords like Wakizashi is compared to 1H euro swords.