I'm currently playing the Guild Wars 2: Heart of Thorns expansion and enjoy it. Since it's recent, there are a lot of players, which makes the big events much more fun. It does cost $50 up front, but that includes the base game as well.
Not really at all a MMORPG but HOTS(Heroes of the Storm) is free to play it is casual and the matches are like 15-30 minutes which is nice. You get to play as a bunch of different blizzard characters.
I was playing PlanetSide 2 for awhile and enjoyed that. I am pretty sure it is F2P too.