Stephens needs to learn how to close. she needs to 'cowboy up!'... or cowgirl up rather...
maybe try to volley/come in once in a while in key games. could do well to look at aggressive players (especially 'closers') on the mens side in particular... federer, sampras, borg? etc... they know how to go for the kill. or maybe even ferrer in terms of keeping pressure on in key points (doesn't choke, ever).
i can't think of equivalents on womens tour. possibly navratilova? maybe Serena, but she has a different style, 'gets all up in her opponent's grill' so to speak. I doubt that it would suit sloane's temperament though.
she has potential, but needs to realise that she can be a 'nice' and still be able to close when needed. it doesn't have to be either or.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM