I'd like Sprite Batching, the ability to modify the style/colour of substrings of sf::Text, and a way of setting an outline for sf::Text.
A Line class would be nice too, I think it was removed from a previous version, I guess for performance issues? Would spite batching resolve those issues?
I'd also like to see more work done on the other areas of SFML, like Audio and Networking, which are generally forgotten. Though I have no specific suggestions in mind. I don't feel like there are any good frameworks out there for audio and networking that are both easy and simple.