This is what I was thinking, that once you had however many copies of something like "Generic Scary Monster," from adventure pack X, you could use them in the scenario provided further down the road with adventure pack Y, so there would be less of a need for a large number of new encounter cards with each monthly release (< 50%). This is also assuming that each adventure pack will come with "for this scenario, assemble your encounter deck like this" instructions. However, this makes me wonder if FFG is looking to make each individual adventure pack a "self contained unit," of sorts, a scenario playable by itself with the addition of the core game. In that case, it seems unlikely that we would see cards carry over to be used in future adventure packs, and that ratio of about 50% player cards and 50% encounter deck cards may stay about the same.
I think what is maybe most likely (purely a guess on my part, of course), is that we will see adventure pack cards carry over and be usable with future adventure packs within the same cycle, for example, players using the "Hunter from Mordor" cards from Shadows of Mirkwood adventure pack #1 later on in the eventual release of Shadows of Mirkwood adventure pack #6. Who knows, though.
According to the French fora, in a post by one of the translators for the game, the way the encounter deck is built is through the icons on the enemy/treachery/objective/location cards (middle-right of each card). if you look on side A of each quest card, there are one or more small icons near the bottom-right, as well as one large icon. So, any cards you have in your total pile of enemy cards that have an icon which will match one of the icons on the side A of the quest cards for that adventure should go in the encounter deck. The large icon (a tree in the Passage Through Mirkwood adventure, or an image of Dol Guldur in the Escape from Dol Guldur adventure) indicates the adventure itself, and it matches up with cards that are unique to that adventure (take, as an example, the Shadow Key; its icon is an image of Dol Guldur, and it will hence only be used in that adventure).
Yer, I'd spotted that too but checking things carefully i think maybe each adventure will be its own self contained unit for a couple of reasons
1. In the example of the key, this gets added to the encounter deck, so as you collect "all" the cards that match the symbols on the first quest card, to prevent dilution problems with the key card, they can't really provide new cards that add to the quest deck going foward.
2. I did think the the orc/goblin encounter cards (The chieftain and Dol Guldor orcs) are examples of those, could be used in a couple of the initial quests (and this still could be the case) including the final adventure in the tower itself, but as the magical stream also has that same icon (a goblins head) and it makes no sense to encounter a stream in a tower, that might not be the case, and so each card may only be used with one specific quest?
3. The description of the expansion packs also says that the encounter cards are "devoted" to the quest. Doesn't neccessarily prevent them being used with other quests from other expansions or the core set, but again they'd need to deal with the card dilution issue somewhere.
The upside of this is that they can make the encounter deck very thematic and really match the quest being played but the downside is that you might get several cards that are effectively the same, but linked to different quests.
I certainly think some cards can be used in more than one quest, even if just from the core set, as otherwise why would they give each quest several different symbols - if the quest had its only set of cards that were only ever used in that quest, they'd only ever be the need for one symbol per quest.
Only time will tell, and perhaps we'll get something in between - perhaps a x number of you orc icon encounter cards and add to the quest deck rather than "all"...? So that they can add cards with the same icon within expansions without causing any card dilution issues