end game?

I have done many tasks in story mode(playing Rihanna) and the last one was fighting off that Jinn woman who wanted to control the world.I destroyed her fleets in all ports and now there's not a single task to do.so does that mean I beat the game?or is it a bug?

Well no actually, I went past that point with Nandi. Jinn is actually one of the few missions before the final for your specific character eg. for Nandi it was Morghianna and saving her from the thieves, or Omar and his brother.

Just go to a random port and find some missions and then check with the first person you gave you a mission.