I don't know if FF has plans to one day add House Arryn (I'd like to see them do so just for the graphic design and artwork the cards would bring us), but if they even wanted to, are there any mechanics and/or playstyles not yet covered by one of the eight existing factions that could fit thematically into a hypothetical House Arryn?

Realistically they will probably just do like what they did with House Bolton in 1st. Ed. and either assign it to another House (non-loyal) or make them neutral.

Please no. There's only 20 cards per chapter pack and we already have 9 factions to cover.

What Grimwalker said.

To answer your question - check out list of MtG keywords. That should give you an idea of card abilities not present in Thrones yet.

i don't think it will happen but it's fun to discuss as a hypothetical. Thematically, my initial thought for House Arryn would be as a defensive house based around fending off the enemy, given the emphasis placed on how impenetrable the Eyrie is. However, this is basically what the Night Watch does, so that wouldn't work. I suppose they could be some kind of control faction, removing challenge icons, removing keywords, blanking text boxes, that kind of thing.

Please no. There's only 20 cards per chapter pack and we already have 9 factions to cover.

As to the topic question, there are only eight slices in your typical pie, so this is really just dreaming, but if I had to assign a theme to House Arryn, it would be control of the Dominance phase, and the ability to mitigate losses due to lost challenges, so that they could leave more of their board standing, allowing more unopposed challenges than most factions. This would reflect their neutrality and reluctance to get involved in the major events of the story, as well as the fact that most of their family dies outside of combat, either through natural causes or the actions of a spouse.

I almost was thinking that House Arryn would have a "remove from challenges" sub theme. In a sense, they have been avoiding conflict this whole time and that is one way to achieve it. Another consideration would be deck drawing manipulation. If they can avoid conflict by manipulating what is drawn(like Bear and the Maiden Fair for Tyrell), that is a round-about way to mitigate conflict from occurring as well.

Please no. There's only 20 cards per chapter pack and we already have 9 factions to cover.

Why 9?

8 Factions plus Neutral. That's nine different categories that have to get coverage in the card pool. Neutrals also probably are going to take up more than 1/9th share, to boot.

I would like to see these as neutral cards. Adding a new faction is clunky. But they could design it so that it sort of feels like a faction.

They could just design the cards so they are kind of meh for their cost, but introduce an agenda that lets us use them at a discount.

Then there are two ways to use them:

-splash a few into an existing deck, if it fits with their strategy (but at their cost, none are an auto-include)

OR

-run the house Arryn agenda and pick a lot of cards which are pretty efficient at their discounted cost and have some synergy, enabling new deck archetype

You could even have some card effects with the condition "if you have a House Arryn agenda" if they want to add some powerful effects that they don't want to be splashed too easily.

Can easily do this for other houses. We already see Lord of the Crossing has the House Frey trait, so they are already positioned to add card texts that care about the traits on your agenda!

They did this when they introduced House Arryn to the CCG. It was a trait that could appear on any card, with the named characters from the book appearing predominantly as neutrals with the House Arryn Trait. There were faction-specific House Card attachments for faction, so that the Arryn trait/House could effectively be played - with a slightly different flavor and feel depending on the faction the Arryns "worked with."

A more interesting option that you haven't considered is something similar to the new Netrunner Deluxe set they have released where they have released three new minor factions that will only ever consist of those cards - so not taking up space in data packs. The balance is that they have allowed those characters the ability to play a broader selection of out of faction cards, to give them more variety.

If they wanted to print house Arryn they could do it but do it in a deluxe box with a few other minor factions, give them a lot of cards and then allow them to use a broader spectrum of cards for deck building through a unique Arryn agenda, therefore giving them variety.

Plenty of possibilities in the business model