There are only a few casting issues. Number one, Tavi is very small for his age until Cursor's Fury and Captain's Fury when he rapidly catches up and fills in. You might need two actors for Tavi. Second, Kitai can be mistaken for a boy when Tavi first meets her, but by Cursor's Fury/Captain's Fury she is described as an exotic beauty. Once again, two actress might be needed, but the same actress could potentially play Kitai and the Vord Queen. Third, there are distinct skin colorations for the different regions. Marat are pale, Countess Amara is described as having golden skin, another region is described as having swarthy skin etc. I personally not be opposed if some of those regional differences are discarded (except perhaps the Marat), but it might upset others.
With regards to Furies of Calderon sets, like Kindler mentioned, they are pretty generic for the most part. Steadholdts are interchangeable, mercenary camp, Marat camp, generic Castle (just have to make sure that this is consistent across all the seasons). The biggest expense will be the Wax forest and the Wax spiders. Even that can be mitigated though by creating one medium set and showing it at different angles to simulate different parts of the forest. The single biggest expense of the first book would be the Marat horde with all their animals.
My biggest fear about a Codex Alera series would be that they would try to save money and cut costs by cutting/limiting the battles. If we are going to get a this series made into a show I want a commitment to show all the battles in all their glory.
One thought I had: if this would get made into a show, maybe they could convince Jim to write a few short stories that flesh out some of the event surrounding First Calderon, the romance between Isana and Septimus, and the death of Septimus. Heck maybe even a little more backstory for Fidelias (what transpired between Academ's and Cursor's Fury) would be welcome. These things are touched on in the book series, but could be slightly expanded and used to pace/fill in the show.
I think it'd fit better as a movie, personally. I've seen what they've done to the Shannara books to shoehorn them into a TV series, and it doesn't work. Like, at all. It's patently terrible in every regard, and I curse the names of those responsible.
Also, in terms of casting, Kitai being mistaken for a boy is an ancillary thing. They can skip things like that, because it's also a little difficult to convey the intricacies of Marat's gender-neutral child rearing for a movie. It's Kitai that's important, not the gender reveal (which was reasonably predictable anyway—writers don't correct someone's use of a noun without it being important (see Ramona restating, "My seven evil exes, yes," after Scott says, "I have to defeat your seven evil ex-boyfriends?" in Scott Pilgrim vs. the World.))
Also, film production takes a while. Kids age. Tavi being small for his age and growing rapidly between Academ and Cursor is fine and all, but casting a fourteen year old kid for Furies is fine when he'll be twenty-something by the time Princeps rolls around. Space filming out a bit so the kids can age, and you don't need to cast the same roles more than once. You've got six movies to do, so it works out pretty well without any special consideration. Physical descriptions aren't all that critical, in my opinion, so long as the characters are
right. Aldrick doesn't need to be a hulking giant, he just needs to be imposing and scary. Tavi doesn't need to grow up huge so much as we need to see him
grow, if that makes sense. As for the skin tone differentiation between cities, it never really mattered in the books, so I don't see it being important in an adaptation.
And as a series... I don't think it'd work well. At least, not as well as Dresden. First, budgets for TV are spread much thinner, and are usually smaller than a movie's budget. It cost five grand each time they showed a vampire getting dusted on Buffy, which is why you only see it once in a while in the first two seasons (you just hear the noise most of the time). It's also why shows like Heroes had most of their potentially awesome fights (Peter vs. Sylar, which is what everyone wanted but never got) be through freaking telekinesis: it's cheap, and you need to stay lean across a twenty-episode season, or you'll wind up needing to do a few bottle episodes by the end.
I also don't think that the books lend themselves naturally to a series. The plot breaks aren't really easy to manage. It's a Two Lines, No Waiting situation, where the story is shifting between Bernard/Amara, Tavi and Friends, and sometimes Isana (though she's often tacked on to one or the other). In Academ's Fury, Isana's working to get aid for Calderon, Tavi is trying to thwart a plot and save the country, and Bernard and Amara are fighting a battle. The action is happening simultaneously, so breaking it up across multiple episodes is rough, and tends to convey a passage of time that isn't accurate in the context of the story. Dresden would be like 24, by contrast; everything is going down in thirty-six hours, and that's clear from the start. But the passage of time during a pitched battle, like in Academ, is dicey to convey correctly if the perspective flips back and forth to Tavi and Isana across different episodes. In a movie, it's simpler, because things aren't interrupted by a week-long break.
And, if I'm being honest, the Codex Alera's length is primarily due to narration. You have pages and pages describing individual fight scenes, which is suitably awesome for a book, but really only takes a couple of minutes of screen time to show. There's a lot of narration describing the benefits of Aleran troop discipline, too, but it's way easier just to show that.
And Bernard and Amara's two-week jaunt through the woods with a certain someone in Captain's Fury can be cut down to about four scenes, total.
It's a different story with Dresden, where so much of the charm of the series
relies on the narration that a voiceover is a necessity, and which will necessitate stretching things out across a few installments. It fits both the urban fantasy stuff—Harry explaining the way magic works, for instance, or the different vampire courts—alongside the Noir style for the first several books. You're not going to get that with Codex, so some things can be summed up on screen pretty quickly.
I mean, I can write a script for Furies of Calderon with a 100-minute runtime, fairly easily, without sacrificing much of anything. I'd even have Gaius do a Galadriel-style voiceover in the beginning with his prologues, just so that you can juxtapose his grandiose ideas with a pan zoom onto Tavi chasing after a sheep.
