I have a friend who just loves gaming of all kinds. He's not a huge Zelda fan (he's played some, but has never finished one), while his favorite series are the likes of Metroid, Devil May Cry, Metal Gear Solid, Resident Evil, and Halo -- maybe you can see a pattern here, visually. Lately he's been playing Dark Souls a lot, and considers it one of the best games of its generation.
Last week while watching someone else play through Wind Waker HD, he said to me, "I just wish Dark Souls looked this good." Considering this is the opposite sentiment many gamers share, and probably the opposite of what you may have been expecting given that first paragraph, this really stuck with me.
Zelda has been given a large variety of visual styles by its developers throughout its over 25 years and more than a dozen major titles. They seem to enjoy creating visually appealing games of all flavors. Most of the games are vibrant and whimsical, and are almost always praised for excellent art direction. Wind Waker in particular is remembered as one of the most visually stunning games ever made.
However, in the wake of Wind Waker HD's release, some are beginning to point to its less-than-stellar sales as proof that more "realistically" styled Zelda games are more popular, especially since the best-selling Zelda game in the past 15 years is Twilight Princess, perhaps the most visually muted title in the series. Some have criticized Nintendo for remaking Wind Waker considering a Twilight Princess remake would almost certainly sell more, and furthermore, many use this argument to support their opinion that the next new Zelda game should definitely aim for a more realistic art style.
To me, this stands in stark contrast to fan reactions to other series. When From Software said they wanted to make Dark Souls II more accessible to appeal to a wider audience, their fans revolted and said the series should stick to its roots. The developer yielded to their demands, opting to continue to appeal to their somewhat niche audience rather than let their fans down for the sake of increasing sales. Gamers lambast Capcom for trying to appeal to the "Call of Duty" audience with modern Resident Evil titles, begging them to return the series to its true survival horror gameplay, even if it means a smaller potential audience and fewer sales. Yet gamers will use greater sales as justification for a "realistic, mature" art style for the Zelda series, regardless of what the developer feels will deliver the best experience. This is particularly baffling, as "realistic"-styled games are already a dime a dozen, while the more stylized visuals of Wind Waker and Skyward Sword are both unique and highly acclaimed.
Mind you, I'm aware that Nintendo could very well make a visual style that is both more "realistic" and unique, satisfying both demands, or that they may CHOOSE to use such a style. But I'm not asking what kind of style you think they should use for their next game. What I'm asking is whether greater sales alone are justification enough to determine the artistic direction of the next Zelda game. If the director and his team want to make something vibrant and whimsical, should Iwata call them and say, "NO. Make it visually darker and grittier and more realistic," just because that would probably sell better?
Also bear in mind that the visual style has NO connection to the maturity of the themes in the game. Wind Waker is as thematically mature as any other title in the series. This is purely about the visual style of the game, not the story or themes.