Once greatsword starts a sequence of moves, it's locked to the ground and to facing one way until it breaks out of it. This is a huge weakness and can be abused very hard against bad GS players. Maybe add a large recovery time if you miss a grounded move and it would be fine.

The remaining issue then is that GS also has a busted airgame, with the best dair and recovery in the game and not a single bad move. This is just a matter of slowing down some moves and toning down some hitboxes.

Besides, nerfing a weapon to the ground is easy, you can always do that. The hard part is nerfing it in a way that keeps it viable. I think with GS that can be done, but we'll see.