Apparantly Natalie Osbourne is returning to Hollyoaks for a few episodes (not sure when), if she is popular with viewers they will keep her on the show permanently. Source: interview with Jo Hallows, Executive Producer.
Also, Izzy is said not to be returning despite recent rumours. Jo Hallows confirmed that there has been no talk between Elize du Toit's agent and Mersey TV.
Hollyoaks are also thinking about bringing Bella back, who has only just left. The show were impressed with the amount of support the character had, and are hoping the actress would like to return.