What's after Black Flag?

Lads, I'm nearly done assassins creed black flag and I was wondering what do I buy next?. Is it assassins creed rogue or unity? I only want to play the main story games so if one of these is just a small side game I wont be buying it. Cheers

I'd consider the DLC Freedom Cry for AC4, which is definitely worth a play-through...

@War_Chiefs: Freedom Cry, the DLC for the game, follows Adewale's adventures after the game. As for the next game in the series, both Rogue and Unity are technically the next games in the series, just during different time periods.