I did not read all the posts so soz if somebody posted this already, but I read somewhere that Colfer was going to end the series on a choice: Arty can be nice and make no profit or he can be nasty and make a milion dollars.
EDIT: I found it. On an interview to the guardian: "One more book, and then that’ll be the end of that,” he said. “He will be faced with a choice where he can be kind to somebody and he won’t gain anything, or he can be unkind and he will find a million dollars in a suitcase, and he will choose the nice way, and that will be the end,” he explained. “That’s how I’m going to finish it, on a very simple choice.”
The child mastermind has changed through the series, Colfer continued, from being “pretty much your standard bad guy … kind of two dimensional”, to someone with “a bit of a conscience”. “He’s gone from being this horrible bad guy to being more of a Robin Hood kind of guy, where he does commit crimes, but you feel they’re kind of warranted,” he said.
Like Peter Pan, Colfer explained, Fowl will put away the magic, the childish things, but he will also do something “that could be considered very human”. “There’s not going to be any huge battle, and going up the stairs to heaven, it’s not going to be that kind of finish.” he added. “There will be the big adventure, but the end will be in a little epilogue. End of story."