I really like Sarah, and kinda feel a bit miffed that we barely saw any of her in-game. Which is fine, I can understand why we see so little of her (kinda hoping the SP DLC sheds a lot more light on the relationship between her and Joel), but being able to change Ellie's model into Sarah's model would be pretty cool in my opinion.
There could be issues such as her clothing not being fitting, or her facial animations stuffing up or something, or maybe even her animations in general not working; I don't know. It's simply a concept I found cool, and wondered if anyone else would like the idea.
Oddly since I ranted in another thread about the lack of decent skins... I think having Sarah as an "Ellie replacer" would just be too distracting personally, especially since it would be Ellie's voice and storyline anyway.
That said, I would have loved to have had Sarah's pajamas as an outfit (skin) for Ellie, as well as maybe her soccer outfit from the photo for Ellie too.
For Joel... how about a 'Six String Samurai' skin, given his guitarist abilities mentioned several times?