(selective reply, I know, apologies)
Guy: I would agree to a lowercase "purchasable," though marketing has given no specific name to features that can be purchased.
NCsoft describe all content on the market as purchasable. or available for purchase. That is very clear I hope from the above and from each of their releases describing new additions.
The closest applicable term we do have is capitalized "Premium", though that one's only true by proxy...
Guy: On the one hand, we could pick capitalized Premium, because it reflects both that the feature is purchasable and also that it is not available to Free accounts. This distinction is important because it draws account type to attention, which is far more likely to pique curiosity and lead people to investigating how they might be able to get more out of the game.
Strongly disagree here, as 'Premium' is a strongly defined term used by NCsoft for a very specific purpose, and never when applied to any purchased content.
Guy: On the other hand, purchasable content isn't necessarily accessible to Premium or VIP accounts, so perhaps the use of Premium to describe those features is ambiguous at best and misleading at worst. Having said that, some purchasable content becomes available to VIP accounts without purchase (such as the Signature Story Arcs and Archetypes), so maybe that's not the best route to go either...
Yes, not the best route to go! Agree very strongly.
To clarify, (and this is trivial), rather than saying "some purchasable content becomes available to VIP accounts without purchase" we and NCsoft would say (I believe!) "certain content is included with an active subscription (or VIP status etc.), but must be purchased by other players" or words to that effect.
I suppose the best, unambiguous trait shared by these features is that they are available on the Paragon Market. Perhaps we should approach it from that angle.