Guns can work, if we can find a role for them, one that takes skill, but you have to sacrifice something for the gun.
It should be a slower reload, but who cares about realism here: Make it IMPOSSIBLE to reload/ shoot anything without an investment,
To avoid how throwing is now. (CQB instakill shotgun-mode while backpedaling.)
If you give up the uber melee-pwning for the gun, you should get a reload speed bonus, and some on accuracy. If you do something monotonously a lot, you will become faster, faster, and faster at it.
I don't think it would f*ck up the gameplay, it just needs it's own place on the battlefield.
Strong, takes a bit to reload, but it should be the rival of the crossbow in middle range.
We have these ugly Side-Sword things. But why don't we have at least an Arquebus with Crossbow-like stats?
Another idea is to make the WPF penalty like how it is with bows, to encourage the typical "Arquebusier" light to medium armour, and make the gun&ammo expensive, so it won't be abused as a sidearm either.
The only problem I see is shields. When gunpowder weapons became popular, people started dropping their shields.
I see how people with their uber Huscarl lolshields don't want guns here. Their "mighty" forcefields would have a real enemy then.