Crossbow as a ranged class is not OP.
If greatswords + regular crossbow is the problem, then make greatswords take 3 slots. So people who want the best melee weapons can't use ranged weapons (except only one slot worth of throwing junk at most).
I rather like this idea. The really upper tier polearms and 2Hers should be 3 slot. Especially for weapons like the glaive and greatswords, they're already beastly killing machines by themselves, and on top of that they have 2 free slots? I call OP. This would also give more purpose to the mid-tier weapons, which currently kind of sit in limbo.
Something else that I think would help the issue would be to increase the slot count of all ranged weapons, BUT make the amount of ammunition they get be based on the weapon and type of quiver equipped, not the number of quivers they equip. Then, make all quivers be 0 slot, allow only 1 equipped quiver, and have that decide the type of ammunition, rather than being an actual object itself. This would promote ranged characters to carry a decent (1 slot at least) melee weapon, which could reduce the number of running archers. (Or at least not give them any excuse to run when clearly outmatched)