Zelda already feels modern, with me playing TP I don't even think of it as a Zelda game due to the realistic graphics and that cannon. Adding some guns to TP's world wouldn't hurt it a bit, that is assuming the gun would have use outside of the dungeon you get it in. I repeat, guns wouldn't hurt a Zelda game if they fit with the graphical style. I would assume Nintendo would use the Wii U Tech Demo style for a realistic-er Zelda, and despite my hate for TP, the game got a lot of success. Nintendo generally doesn't fool around with innovations, as seen in TWW's cartoon graphics, TP's realism, SS 1:1 swordplay, and so on.
Having guns wouldn't detract much from the Zelda Experience so long as it doesn't outwardly replace the sword and shield. And I repeat again, Zelda wouldn't become an FPS just because there are guns. Don't cite Call of Duty unless you've actually played the game; having been a CoD geek for a good while I can say that throwing in a gun and giving us the ability to move and shoot would not make Zelda a terribad series. Fable, which is oftentimes cited as the Zelda for the Xbox(360), has swords, magic, and guess what? Guns as well. That series is still thriving. Innovation and renovation are always seen in games across the world. Most of the time, the innovations are pretty good, assuming the company which made the game knows what they are doing. Yes, Zelda + Guns would be different, but so many Zelda games are classified as different yet still succeed. I think if all of the 'hardcore' Zelda fans would open up their minds a little bit and at least try other series, they wouldn't be so turned away from 'radically' different ideas. I'll try to show the changes Zelda has had over the years:
RPG ELEMENTS -
Zelda II:Adventure of Link basically deviates from all that we call Zelda. It's an RPG with blatant experience points for crying out loud! Did it succeed? Yes, it did. People hate its guts, but that's because they (meaning me) cannot beat the game without cheating. Succeed it did never the less.
EPIC STORY -
Zelda:A Link to the Past changed the way people viewed game's timelines what with the game having an actual (back)story to it. ALttP is the favorite of many people to this day, having garnered extremely well reviews.
Zelda:Ocarina of Time brought the Zelda series into the realm of 3D gaming. Needless to say, that game got the best reviews of the series to date.
DARK STORY -
TLoZ:Majora's Mask was undeniably twisted and dark. As a direct sequel to a lighter game (i.e Ocarina of Time), seeing something so insane as a boy in a mask trying to bring destruction to a world of happy people is rather different. Did MM succeed? Partially, but it still has a thriving fanbase today. Its partially success is due to being released at the end o the N64's life cycle, I should have you know.
The Wind Waker with its cartoon-like graphics is probably the worse offender, but Twilight Princess also did a change with the realistic graphics. Both of them turned a couple of players off at pre release, but once they got their hands on the game a huge fanbase (of mostly fangirls but whatever) sprouted from things such as LinkxMedli or LinkxMidna to 'crazier' things like Wolf LinkxIlia. Those are only fanfiction pairings that people do, reviewers around the world praised the game's graphics and sound, and the gameplay as well. TP and TWW both succeeded a great degree, that's undeniable fact.
Skyward Sword, which is due for a release of November 20th in North America (18th in Europe I believe), clearly shows what 'radically' different gameplay can do for the series. Will it succeed? Time will tell, but from what we've got from pre release reception makes the game sound like it could be the next 'Ocarina of Time'.
This is Nintendo we're talking about, and we're dealing with a prestigious series called The Legend of Zelda. I highlight the word LEGEND for a reason; legends of all sorts happen whether they are modern or not. Zelda is getting modernized every day, be it through gameplay mechanics or story. Having guns in a Legendary series does not render it Un-Legendary. Look at Call of Duty. The series pumps out a new game EVERY YEAR, and MILLIONS of people buy the NEW GAME EVERY YEAR yet all it does is ship out the same multiplayer mode with hardly new guns and somewhat new perks. The game makes millions! Transfer one of the main selling points, that is the guns, to Zelda and you have potential for a great game. Does Guns + Zelda: FPS? No, it does not. We have a bow and arrow, but it doesn't make the game turn into an FPS (we have a choice between L-Targeting and FPS aiming in most games). Guns wouldn't be any different. You won't know until you try, and generally when you try, you get sucked in as evidenced by food, movies, books, any form of enterainment 'round the world.