Steffi CAN'T come back, regardless of how much we would love to see her grace the tennis court once again.
I read about 9 months ago that she now has pretty serious pain in her legs whenever she tries to run or train. Basically, that last run in 1999 took out of her everything that she had left to give.
As I remember, she wasn't able to complete - or barely even start - her exhibition tour of the world in 2000, having retired the previous year.
I doubt, therefore, she'd be able to come back after all this time considering the state she was in only two years ago. Besides, she's got a young 'un to look after these days!