The loss to Siegemund doesn't look as bad, given that she made it to the final. Still, Halep is not going to be my pick to win Roland Garros. At this point, I am leaning towards Kerber (deep in Charleston, won Stuttgart). It'll be a much easier pick if Azarenka is on the other side of the draw; she could win, but she's only won one clay singles title in her career, back in 2011.
As for Serena, well, all things are possible with Serena. It wouldn't be shocking to see her win. It also wouldn't be shocking to see her lose early.
Perhaps Halep needs that sinus surgery that she's delayed.
Let's see if Djokovic rebounds in Madrid or Rome. Wouldn't be surprised if he lost to Federer in Madrid (if he plays, though Federer has traditionally played well there), but a loss to anyone else would be bad news for his Roland Garros plans.