Optimally, we would break down the weapons table and assign values according to usefulness. Like downgrading the damage die if it has this trait, upgrading if it has this one, etc. Trying to figure out a balancing point in weapons for 5e is rough because the weapons as they are are not balanced. The classic example of this is the trident vs. the spear where the spear is identical in combat, but has better cost, encumbrance, and is simple. It becomes a bear because our main criteria for something being OP is whether it is across-the-board superior to other options of its type.

That being said, the “falchion” presented seems like a great common magic item-that-isn’t-magical. It is absolutely OP as a weapon on the table because it is a greatsword which becomes a better longsword when one-handed, that’s compatible with GWM and whose only drawback seems to be that Small races can’t use it.