Well, we have actually had a gun in Zelda. What is cannon but a large gun? We've seen explosive powder cannons in at TP, WW, SS, and ST. And what are bombs but a gun with no barrel or shot?
Now I don't want any sort of actual handgun or musket taking a prominent role, for that would ruin the feel of epic and classical warfare and adventure. I do however like the idea of cannons seeing a larger use in a future game. They would ruin LOTR, but this series has seen trains, cannons, rock bands, submarines, and modern-looking bars. It's hardly a step up. It would certainly be cool to storm a huge ship packing multiple cannons. It could make for an epic battle. Now the people on the ship could have bows or swords, but I'd like to see Link storm a huge pirate ship or fortress bristling with cannons. Now link could never touch a hand-cannon or gun, but it might be neat for some huge iron knuckle or something to have a cannon for a head or maybe a cannon with legs. I'm all for diversifying, and people taking these games more seriously might benefit the franchise. Granted, these cannons couldn't dominate every section of the game, but they might make a nice touch to a less whimsical game.