Keira won't return, she hasn't been anywhere near the set. This episode 5 will have a lot of links with episode 3 (At World Ends) though.
Elizabeth's son Henry Turner (Brenton Thwaites) who grew up in the South China Sea with his mother (remember Elizabeth became Pirate Lord of the South China Sea and had her own ship called Empress), Henry will go on a quest to find his cursed father who replaced Davy Jones as the Immortal Captain of the Flying Dutchman.
The movie will focus mostly on that but also Sparrow VS Salazar.