My suggestion here is pretty obvious, make items that you cannot store on your person only take up one slot. Example is a pike, a lance or even a flamberge. These are of course large weapons, but you cannot store them on your person, if you want to use a different one, you have to drop it. If you have to move, it'll mean leaving it behind.
If archers bring flamberges, they drop em to shoot and get run off, they are left without their flamberges.
Apparently the slot system is to hurt the swiss army knifer characters. Fine, this suggestion will still nerf them. No longer can you come across an archer who suddenly draws a pike or a gigantic sword from out of his ass. He might have one lying around, but he will have to go pick it up or if he has moved, he has to look for it. This means it gives the attacker an advantage over the swiss army knife player. At the same time, it will make these kind of classes less mobile. An archer can now bring a pike, but he has to stay around it, or pick it up and carry it with him. This makes him much less mobile compared to those that are carrying a sheathable melee weapon instead. Maybe it would lead to more camping, maybe it would lead to more realistic playing? I can't tell, might be cool to see though.
This may seem unfair to polearms I admit, IRL you can strap a pretty large sword to yourself, but polearms are usually a bit longer.