Security improvements are set to be a big element of the new update plus there will also be a new default blue and green wallpaper on the homepage.
A relaunched Beats Music Streaming service is also expected to be a big announcement at WWDC but some rumours suggest the software isn't ready just yet.
Block rockin' Beats
Billboard is reporting there are key music licensing deals still missing from the service but a source said, "If any company can pull it off, they can" as "labels are more likely to play ball with them."
Meanwhile separate sources have told 9to5mac the release is set to go ahead and that new design changes to the Music app within iOS 8.4, such as tweaks to positions in the lists, signpost what Apple is planning.
Those sources claim Apple will be launching the service in a long list of countries in late June but is yet to share which countries will be included.