I always thought Clare King would have been perfect as Derek's ex wife.
In todays show I'm not sure where she would fit unless she possibly played Elaine, Linda's mother? It would mean Elaine would have had to have been a young mother too but it's possibly and I could definitely see them working as a duo.
Elaine would definitely be a different role for Clare too as it's pretty clear that she's quite a clingy parent but you can certainly imagine the kind of chemistry her and Danny Dyer would have as it's been suggested that Elaine and Mick really get on, much to Linda's displeasure.