In my opinion, the games are best as they are. There's already a ton of implied romance, and the wonderful thing about the series is that you can often read into a romance or a lack of romance yourself. It goes along with the "Link as a link between game and player" mindset that the protagonist was named for. I mean, look at all the love interests Link's had! You've got Zelda from pretty much every single game, Marin, Malon, Saria, Ruto, Nabooru, Navi, Ilia, Midna...and those are only the ones off the top of my head that are strongly implied. If you start looking into things, there are a multitude of characters you could imagine a romance working with. While there's never anything overtly explicit, it works better that way. You can read as much or as little into a relationship as you want, and you can also read as much or as little into Link's reciprocation. It's ambiguous, and it lets the player decide how much romance is in the series. One person may view Ilia as nothing more than Link's childhood friend, while Midna and Link are clearly made for each other. One person might interpret a much more romantic bond between Link and Ilia, and Midna? She's just a manipulative woman flirting with Link to attempt to get what she wants. Maybe Ilia's just a friend and Midna's just manipulative, or maybe Link's caught in a relationship between the two of them. All four of those options have strong arguments, and blatantly hooking Link up with one or the other would have probably upset a few fans. Even in Spirit Tracks, where there's really no romantic option other than Zelda, the relationship could be interpreted as either romantic or platonic. Link's Awakening, on the other hand, pretty much had Link/Marin canonically established, but that's the only one in the series.
The romance in Zelda is, for the most part, very innocent and pure. The romance between Anju and Kafei is possibly my favorite video game romance ever. There's some fanservice and a few suggestive lines, yes, but it's nothing that would, on its own, get the game a T rating. I personally think that the series is doing fine as it is.