I had toyed with the idea of Biff changing but I quite like the idea of having at least one male on "male" relationship.
In regards to Lorraine we could try one of the many suggestions that I gave earlier. Although I quite like the idea of Lorraine becoming Steve "the stud" or maybe one of the football players.
Also with Gary I would like to keep him as he is for now but if he were to change again we could do one of the suggestions or maybe he could become Tiffany or one of the cheerleaders. Just my thoughts