I wonder why nobody has done something like this yet?
Does it have any bad consequences to stack things like first aid kits or metal plates and such?

It seems to stack just fine and you can use the items or unstack them by drag and drop.

Open FCS tool and change the stackable value for items you want to be stacked.
Save your mod and load it in the game.

Well, certain backpacks will allow you to stack items as it is, and I think there are mods to increase that capacity already.

Still, I've no doubt some will appreciate this.

Yes, it provides the same ability to stack like vendor's inventories do, up to 4 items.
I made this 100, the weight is the limit :P

That's cheating obviously, cause it's a game about traveling and this means packing bags. And here size matters, not only weight. You should consider using animals/backpacks if you can afford them.
I'm just trying to get this picture of a player with a hundred sleeping bags out of my head ... reminds me of how my car looks on holidays ;-)