I am hoping Donna will be present because, though she is intimately tied to the Laura arc, I feel that she was a character valuable on her own and intrinsically able to function outside of Laura (though her best self is when Laura hovers in and out of her stories).
Even then, thinking about it from a realistic point of view, Laura was Donna's best friend since childhood and the two were nearly inseparable throughout the formative years of their lives. Once Laura was murdered, of course Donna felt a shock and devastating life change. And while (25-27 years later) yes, Donna might have moved on with her life... her best friend was still murdered brutally/ritualistically and even darker than that, by her own father. There is absolutely a way to incorporate her into The Return organically through this.
So what I am hoping for is that Donna will be back but played by a new/third actress. That way it is indicative of a larger nature to the character (Donna=Mercurial) and also does not leave either LFB or Moira Kelly as outliers. If LFB returned or if Moira Kelly returned then whoever did not return would become an exception to the rule, a strange and noticeable diversion. Up until now that wasn't a problem because there was only the Series and FWWM so two separate Donnas for two separate entities is fine, but once you add a third in...
So a new actress would establish a pattern or rule that Donna is fluid and malleable, taking a different shape to fit the different worlds of the different iterations of the saga. And that would be the best and most beautiful option to me anyway.
On a semi related note, I know we all love Gersten and are excited for her to return but I gotta show some love to Harriet Hayward! I really wish she would have returned for the new series too...