Oh my, yes. XD So much we were teased with. (And I get the same impression about the AF series in it not being planned from the beginning. Especially if you read author interviews. Not sure if you've seen any of the pre-TAC interviews, there were some interesting ideas there, but the actual story of TAC was quite a bit different. [It also seems clear that Colfer would have liked to work H/T into the series, from as early as TEC, but it never materialized.])
Anyway, some of mine are these:
1. The time-stop. We only really saw this used in book 1 (unless you count
I would have liked to see it again. (And we never did find out if the fairies ever figured out how Artemis escaped the time-stop, so that thread was kind of left dangling.) On the one hand, I suppose only appearing once gave it more power, but it was one most the interesting concepts of the first book. (Also still confused about the rules surrounding the time-stop. Apparently no one has ever figured out a way to escape the time-stop [besides what Artemis did in book 1], but it explicitly says at one point that Foaly's created a portal that lets you leave whenever you want. (What?? I'm still trying to reconcile that one.)
2. The Book. Again, it plays a huge role in the first book, and it seems like such a big deal, but in later books it's only mentioned briefly here and there. (No1 undoes the spell that keeps fairies from entering dwellings like it's nothing, but wasn't there something in the early books saying that undoing the spells wasn't approved by the courts? As it would be violating the ancient sacred laws. I always wondered just how seriously the fairies took their sacred laws, they seemed less strictly enforced in the later books than the earlier ones.)
3. Second your wanting more description of Haven and Atlantis, and all the lower elements really. For Atlantis, as far as I remember, the main characters never actually went inside the city, even in TAC. (We only ever saw the prison.) I'd like to know more about what it looks like, and how the people's customs there differ from those of Haven and such. We also know that Haven is the biggest fairy metropolis, with Atlantis possibly being the second largest (maybe? I can't remember, but that's what I've always assumed), but does that mean there are other smaller fairy cities scattered about? For some reason I've been picturing it more like small settlements scattered about, and some individuals families living in various places, in a sort of frontier-esque way. I know we saw the abandoned theme park in TOD...where was that again?
I like your idea of fairy city-states, and even perhaps skirmishes over the centuries...the People always seem to think of themselves as a peaceful race, but you see disgruntled officers like Cudgeon teaming up with maniacs like Opal Koboi and plotting to overthrow the government, and goblin uprisings in the capitol city. Seems to me like their were probably more all-fairy conflicts than they'd like to admit.
4. TLC, when they see the dimensions through the rips in time (while they're on Hybras, and the time spell is breaking down.) Are there other dimensions with people/creatures living in them? What kind? (Parallel worlds possibly?)
5. Foaly's space probe (TAC), and other mentions of space and the possibility of there being aliens. But maybe this is just because I like stories with aliens. (Animorphs, anyone?) For some reason I always liked the picture of a race of evil aliens out to conquer the galaxy coming to earth, forcing humans and fairies to work together... (Yes, apparently I go for the cliches. XD!)
6. Gremlins. Mentioned, but never described, and you never see a gremlin character. (What kind of people are they? What's their culture? What do they look like??)
7. Quantum Zombies? Eh?
8. Mud-fairies, or humans with magical blood, the result of human-fairy unions. I can't remember if this concept was ever mentioned in the books, but Colfer's mentioned it in at least one interview, and I think one of the codes from the books talks about them.
9. Section Eight. They were a secret part of the LEP which existed for centuries, watching for demons and carrying out high-level missions for the LEP on the side. We know (I think?) that they switched over helping demons get reintegrated with Haven society, but I'd have liked to see more of them. (And what happened to them after Vinyaya died?)
10. Also, totally second Minerva. She's was there for one book, then disappeared, never to be mentioned again. She's another human who now has knowledge about the People, and TLC left off with the definite impression that she and Artemis would have more interaction in the future. There was a lot of potential there. (Skier, egh, I know. X3 Though I suppose if Artemis goes for a police officer, who knows what's possible. But that 'when she saw this guy ski she forgot all about intelligence' thing. No, please.)
11. Qwan. He was an interesting character, but aside from a few mentions from No1, he doesn't appear after TLC. (In fact, he's almost written off by a line that says something about how No1 'pretends he didn't pass his teacher in skill years ago.' Yes, No1 has incredible magical powers, but Qwan is a magical master and expert on the equations of spells and time travel [which is on par with rocket science in terms of complexity] with most likely hundreds of years of experience, plus his thousands of years as a statue, where he was apparently conscious.) What is Qwan doing now besides teaching No1 and Qweffor? Seems like his first-hand experience with the ancient war with the humans would make his knowledge of great cultural value.
12. You mentioned telekinetic fairies, I wondered about that too. XD The series also mentioned empaths, which, if I'm remembering right, are fairies who are able to tap into a collective memory of the events down through the ages, or something weird like that. (Or able to sync up with other warlocks and get their memories before they die. Something like that.) I thought that sounded interesting too, but we didn't learn much about them...
13. I wanted to see more about Foaly's app 'Offspring.' XD! (Was that a real thing, or just Holly's oxygen-deprived imagination? And why would she be coming up with something like that anyway?)
Juliet: Yeah, Colfer did keep talking about wanting to bring Juliet back into the story, but he didn't really until TAC and TLG. (She gets back her memories in TAC and fights zombie wrestling fans, then in TLG she
But I don't think we really see the same character development (at least from what I remember) that we saw in TEC. I'd have liked to see her come back and play a larger role as she did in the earlier books...
The Russian Mafia (and other mafias): Oh my, yes. I totally agree. X3 (We really get an image of what Artemis Senior is like from the early books, and that vibe of cold, ruthless criminal empire kingpin is really there in spades. So when he comes back from Russia a changed man, you'd think that past (and while we're at it, Artemis's past, too) would come back to haunt him. Opal Koboi comes back for vengeance on Artemis every other book, but it might have been nice to see an enemy from Artemis/his father's shady pasts, someone who they might have even worked with.
Space program: I know. I think it was first mentioned in TLC (I think that might be where it first talks about how Vinyaya led a team to the moon, if I'm remembering right), and it sort of keeps coming back, but we don't get a lot of concrete information on it. In TLG, No1 goes to the moon on an expedition
because if he'd been on earth, he would have inconveniently been able to put a stop to Opal's plot.
But we don't know much about it. (Again, I want to see an alien plot twist. ;9)
The C Cube: I know, the idea of an unbreakable 'eternity code' is so epic. Though I think Fading is probably right that when Artemis was mind-wiped, the fairies would have destroyed it...although, I don't know if an eternity code would be obsolete even by fairy standards. (I could totally imagine Foaly still has the cube sitting in his lab, and he occasionally goes back to look at it, to try to crack the code, and is still working on it even now.)
Doodah Day/Mulch detective agency: I know, I was disappointed by that too. X3 (I mean, Mulch had come such a long way, and then we come to TLG and he's back to where he started? What??) I've wondered where Doodah ended up. (Yet another character of TLC who was relegated to the 'never to return' pile...)
Also, so many plot threads left dangling in TLG. X3
Oh, and there's one last thing I used to really want to see addressed, but it sort of faded to the back of my mind when I read a fanfiction that did it already. ;9
At the end of TLC, after Artemis essentially brings Holly back from the dead, he thinks to himself something like, 'There will be consequences for this. You can't change the past and go unscathed.' (Something like that.) At the time, I so wanted to see something come of that, but in the end he essentially gets away with it. X3
There's probably more I'm not thinking of, but that's all for now. ;9