I have just gotten the core game for the Game of Thrones LCG and I was wondering if it is possible to build a Greyjoy and Martell deck from the Kings of the sea and Princes of the Sun expansions, that is similar in theme and power to the four decks in the core game pack? That is, it can contain most of the well-known characters from the faction and it is not gonna steamroll the other core decks (or be steamrolled by them).

The reason is that I am not gonna go into the collection and deck building aspects of the game, but I would still like to have a deck for all the factions.

Oh and one more thing, do the expansions come with plot decks that fit with the Houses, so I can build a plot deck for the two factions?

There are 7 plots in each of those boxes, including 1 House specific plot. I don't think those plots are enough to make a plot deck, though, because you'd be missing a reset plot (like Valar Morghulis). As for House decks, you might be able to create a functional deck for each (Martell and Greyjoy), but it'd be harder than with the Core Set + House expansion for any other House.

I have played a LOT of games using the Core set decks and using the single packs of PotS and KotS and I would say in melee they can stand up to the other houses. I've read were people add or take away cards, but honestly I haven't found that necessary. KotS is a bit easier to play than some Houses (similar to Stark in direct approach) while MArtell is probably the hardest to play (a number of their cards involve losing a challenge or require a bit of info on knowing when is best to trigger an effect), but the power level is comparable. For PotS if you cut out the out of house cards you will pretty much have a playable deck, with KotS cutout the kingsmoot title cards and you are good to go.

Neither plot deck really needs a reset in the same way how two of the Core set decks don't have one, when you are talking about adding them into that basic melee setup. If you are planning on playing joust (1v1) then I would certainly recommend slotting in a reset plot, but I recommend that for the CS decks without one also.

Ah cool, I was planning of running mostly melee with my friends and just wanted to have some more options without breaking into the whole collection-buy frenzy, that way lies only madness...

I have been playing the Call of Cthulhu LCG and got an almost complete collection (with three copies of every card) up to the third pack in The Yuggoth Contract cycle, until I realised I was the only one in my group of friends interested in deck building, the rest just wanted to play a Call of Cthulhu card game and not be bogged down in choices.It wouldnt be so bad if there was a LCG community here in Denmark then at least I would have someone else to play constructed decks with, but Magic and WoW TCG is all there is to be had. Ah well, this time I will keep it simple :-/