Agreed, as this is just the first book (possibly incorporating the second, too) there should definitely not be any hint of romance between Artemis and Holly. But I don't think there should really be any danger of that unless they really make Artemis quite a bit older—(Artemis is like twelve in the first book, and even if he was in his late teens, I don't think the moviemakers would want to deal with the potential backlash at hinting at romance between an adult and a minor.) I'm banking on that anyway. XD!
But I know exactly what you mean, they do that quite a bit. (Like, in the Ender's Game movie, I feel like there might have been hints between Petra and Ender, which there were absolutely none in the original book, because Ender was like eleven, and Petra was older. Though maybe I'm just imagining that, it's been a while since I saw the movie.) I also watched the first Percy Jackson movie just recently (with everyone saying it was so bad, I was curious XD), and they added in quite a bit of romantic overtones between Percy and Annabeth, whereas in the books, I don't think that started until later in the series.
I think one of the things that will make or break the AF movie is if they're able to capture Artemis and Holly's relationship as friends (or, I suppose, enemies then almost-friends in the early books), while keeping from going in a romantic direction.
Yeah, a too-feminine Holly wouldn't fit her as she's portrayed in the books. But she's not a totally tough she-man either (note her comments in book one about using moisturizing to keep her ears from flaking); I've found she can be a really hard character to get right. (In fanfiction, I find portrayals of her character that really match how I see her pretty rare.) But I think there's a lot of directions the moviemakers could take Holly, so I'll be interested to see what they do. I think I'd rather see them go a little too masculine as opposed to too feminine, as as you said, that kind of portrayal would be really diametrically opposed to her her spirit as a character.
Another thing I wouldn't want to see is a movie that focuses too much on the humorous aspects of AF; that is, make it a movie that pushes it toward the humor genre. I like funny movies (certain Disney movies like The Lorax come to mind XD), but I would rather see an AF movie that leans more toward its spy aspects, with a bit of a dark feel, and awesome special effects when it comes to the magic and technology, with the humorous bits laced in. (Maybe toning down the stuff with Mulch. XD My dad keeps telling me the real reason they keep postponing the movie is that they can't figure out how to get Mulch fit for the screen...thanks, dad.)
All that being said, I really just want an AF movie. (I'm predisposed to love it no matter what they do, whether Artemis is twenty and Holly has luscious movie star hair.) I think what a lot of people didn't like about PJO was that it was so different from the book (characters were made to be teenagers instead of twelve-year olds, and the movie seemed more geared toward a teen audience than the original book, Annabeth had brown hair and romantic aspects were added in between her and Percy, and quite a few events of the books were switched around and not the same, and acting felt a little awkward at the start of the movie too, for some reason), but I thought it was enjoyable overall, and I'd be happy with an AF movie of the same level of production and similarity to the book as that was.
“After all, absolutely no one can help but suspect a criminal, liar, and manipulator of committing crimes, lying, and manipulating. And of course, no one is more aware of that simple fact than Artemis Fowl.”
Opal sets into motion her most diabolical scheme yet, to frame Artemis and turn his closest friends against him. Only this time she has a new calculating partner who knows Artemis better than he knows himself. [Post TAC] https://www.fanfiction.net/s/8336552/1/Noble-Heart
...Shameless self-advertising, guys! C;
(And if you're really bored: http://axxonu.deviantart.com/gallery/28912232/Artemis-Fowl
AF fanart. ;J)