I would stop reading Colfer's books immediately if it ended with a kiss. It would be analogous to Twilight in this sense. Artemis Fowl is simply put, not classified as part of the romance genre. It is a crime/fantasy series. For the male readers, which I would assume, is the majority, this would be just ridiculous. Even if half are female and the rest are male, an ending kiss would ruin it for the guys, therefore making the target group much smaller. A different ending, perhaps some type of cliffhanger would probably make more people happy. It shouldn't be an entire story waiting for us, but something for fanfiction writers to think about.
A good example that would make many happy, and allow fanfiction would include a plot to wrap the story up. Perhaps peace between humans and fairies. Maybe Opal dies. Or even Artemis. It might end with some romance, but not a gooey romantic type one since that wouldn't fit in at all. The best way to fit it in would be a note left on the doorsteps in the main foyer of the Fowl Manor or a message on Artemis' answering machine. Maybe one on Holly's; something that causes a revelation in one's head that reveals feelings that were once hidden. It caters to all, creates a good base for future books or fanfiction and wraps the main story up.
Lol, you know, there are probably boys who would like the kiss thing and girls who wouldn't. Just to clarify. (;
But I agree, I like your idea. Though it wouldn't necessarily have to be a note or a message, I like the idea of something that causes a revelation... and fodder for fanfiction is always a plus. That would keep people interested long past when the final book came out, which would be great. Actually, I confess that oftentimes the series that I like the most aren't about what's actually in them, but about what captures the imagination that makes me want to imagine new stories about the characters. When there is too much romance, I may enjoy reading it, but once the book's over I put it aside. When there is only hints and bits and I have to imagine the romance out for myself, that's when I get more involved.
Ooh, and the peace between humans and fairies part, that's a secret hope of mine. Though sadly I don't expect it to happen...