Well the fourth one went way off of the plot line of the other three, I think. In addition, the forth one also lost some of the best actors including Orlando Bloom (Will Turner) and Keira Knightley (Elizabeth Swan) so in the fifth movie, I personally think that they should bring Orlando and Keira back and get back onto the plot line of the first three, but my opinion is probably a very unpopular one, so I'm sure they'll continue what they were doing in the fourth one with the mermaids, magic, and what not. I just wish they'd return the story to what is was originally. I think before the fourth movie, Pirates of The Caribbean was a much more original idea.
I honestly didn't like the fourth movie as much. Don't get me wrong, it was a good movie.
It's just, I feel like the plot ended with the end of the third movie. They also lost a bunch of classic characters when their actors didn't return for the fourth film.
So, I don't know how the fifth film will work. They kind of killed off the villain too in the last one (Blackbeard). I don't think the actors who left will come back. And where would the fit in? The crew is either dead or stuck on the Black Pearl still I think, Will is sailing around helping the dead souls, and Elizabeth probably has her son by now.
Oh, she does have her son. In the end of the third movie, mid way through credits, it showed how they lived and how Will would sometimes return to see Elizabeth and their son.