I don't know why anyone one would think guns would be a gameplay addition, consider this:
Muskets(and all other primitive rifling) were possibly the stupidest invention in human history. They jammed alot, if the powder got wet they wouldn't fire, they were not very durable. They were not very effective(poor accuracy, wouldn't do much good against someone in armour), Which is why some idiot generals decided to take away the armour and line everybody to take turns shooting at each other.
Last, I'd like to point that it took a trained soldier an entire MINUTE to reload.
Now, consider these points, how is this going to implemented gameplay wise?(I know its NOT, but think for a second).
Armor disappeared because as guns improved the armor became more and more obsolete. Even the first guns could pierce armor quite readily. I don't know why you think it didn't do much good... They lined up without armor because the armor was pointless by then and because that's how large battles were fought. It was also the only way to get a good effect on the opposing force. That rapidly changed as guns became more accurate, reliable, and smaller. You see this happening before the Revolutionary War.
It's also not the stupidest invention. Even in their infancy guns had a place on the battlefield.