It wouldn't really make sense for Donna to return. Her relationship with Marlon was dead by the time she left, and her entire family, except her two small half-siblings, is gone from the show. (I'm still bitter about them killing of Viv by the way). Of course that won't stop them from bringing her back if they want to.
The one person I would have liked to see back, if I was still watching, would be Robert. That he wasn't there to support Victoria when Jack died was unbelievable.
Another blast from the past they could bring back could be Sam Skilbeck, son of Matt and Dolly. He left the show with his mother when he was only 8-9 years old, so he would be a blank canvas, yet at the same time a link to the old Emmerdale. He would be in his early thirties now and could have a family of his own. With him back they could also have Matt and Dolly make the occasional appearance, but only if Frederick Pyne and Jean Rogers played them. No re-casting thank you very much. Ok, Katharine Barker would be ok as well for Dolly since she originated the role before Jean took over.
But this will naturally never happen. I doubt they even remember the Skilbecks now.