Personally, I have no interest in the coop LCG games. For me, the challenge that makes the game interesting is to beat an opponent, someone who has their own deckbuilding and game playing skills that I can pit my talents against. Playing against a simulated opponent who doesn't adapt or have any imagination is going to get old fast. So, I'm hoping if another LCG comes out that it will be a regular two player competitive game.
As far as theme, I have no idea what it might be. For me, the main point is that the game has to be good, and theme is secondary. An attractive theme won't get me into a bad game, but a bad theme might be able to keep me from investigating a game enough to tell whether it's good or not. For me, A Game of Thrones falls into this category. I find the theme dull and uninteresting, and am disinclined to pick it up. And yes, I've read some of the novels. I think I made it through about 3-4 of them before giving up and I couldn't tell you much of anything that happened in them either.
Personally I don't really care if it's a well known license or just a general sci-fi, fantasy, etc. theme. Nothing wrong with making up a new theme as far as I'm concerned, you've got more control over it and fewer limits placed on you by the source material. Neither Magic nor Legend of the Five Rings were based on existing themes, and they're doing just fine.
It seems like the best fit for a competitive game is one where there are multiple sides at war or competing against each other, so I'd start by looking for themes like that. These could be countries, kingdoms, rival mobs, alien races, kung fu temples/styles, schools of magic, or whatever.
Some sort of weird WWII might be interesting, but I like the idea of rival gangsters or kung fu temples too. I'm up for anything really, so long as it's a competitive LCG and has a decent set of rules and an interesting story to it.