Having seen both Frances Ruffelle and Lea Salonga (along with countless others) in the part, there was a reason Ruffelle originated the part. She has a truly unique voice (of which I am a huge fan), and her aching undertone in her songs is heartbreaking.
As for her playing Kim, she is a bit too old now, and not quite the same ethnic quality needed for the role. I remember her saying she would love to do something like 'Mimi' in 'Rent' a number of years ago. I would say our most likely chance of seeing her on the Broadway boards would be if she came over her and repeated her 'Roxie' in 'Chicago' (which she has done a couple of times in London). I am not sure she would be up for an extended stay though, as she has come across as someone who puts her kids before her career.
My mother always used to say, "The older you get, the better you get, unless you're a banana." - Rose Nyland