Well, you wanted simple, it would give different strategies from HS, and maybe mean a better chance to not be called up by Blizzard's lawyer team.

Maybe something to consider for making Undead Lily different from this?
Anyway, another suggestion. You don't have to worry too much about balancing, since this isn't a PvP game, so you can definitely buff the AI. Now, you can of course have cards that only the AI can use, and I'd expect to see some, but ultimately it's better to give more options to the players than to take cards away from them, and you have a limited card pool restricted by the amount of artwork.
One way to buff the AI could be to boost existing cards, let's call it the "Medal" system. Say a medal gives a card +1/+1 attack/life. Now the AI can have the same cards as players, but some/all of them are buffed with medals, raising their stats and maybe even giving new abilities the card didn't have before. This would also be a simple way to have a difficulty system, the amount of medals on cards increase with higher difficulties.
A nice developer would allow players to earn medals too (feature creep, I know, but doesn't seem excessively hard to do), which we could assign to cards of our choice. So, if we wanted we could buff a cheap unit for early advantage, a huge unit for extra beatdown, a flyer for more unavoidable damage, etc.
To make this a really interesting addition, medals could be earned through optional objectives, such as winning a fight with more base life than you started with, dealing 15 damage to the enemy base in a turn, or whatever.
I'm just putting ideas out there.