There's a certain Reddit subforum dedicated to compiling leaks regarding G5, and from what they've compiled, it sounds as if we will have about 10 years (give or take some months) of G4.
There's (allegedly) discussion within the writing staff about how to handle the new show, how to make AJ more popular as a character, and some possible designs have been leaked.
It's unfortunate that Star Dreams never got into the show as much, but it is highly unlikely she'll have an incarnation in G5. Aside from Blossomforth, early brushables that weren't show-originals got swept under the show's rug in favor of new characters.
I'm hoping for a turnaround in the quality of the toys with larger variety in character offerings, but I'm sure that other show minor characters (my fave is Silver Spoon) just won't make the cut. It sounds as if they're going for a more fantasy/adventure setting rather than a slice of life, and there's not really too much room for a bully's sidekick.