Some people won't agree with me but I really think that Justine's best surface isn't clay but grass.
I think that Justine needs to convince herself that she can win a big event on grass.
Her game, like Roger's is especially suited for grass, she has a good serve (when it lands in). One advantage is that on grass you don't need to hit a 115 mph serve to win free points, the surface helps, so she can take pace off and increase her percentage. At the US Open, despite her terrible form the serve seems to have improved, maybe because she practiced it more during her off time because it requires less (physical) effort.
She has the volleys, she has the slice, the one-handed crosscourt backhand can be lethal on grass because of the angles (ask Jennifer).
Said that, the problem with grass is that there are not enough grass touranments, sure all players face the same problem but some adapt their game quicker than others. Serena for example doesn't need to change her game too much, her serves and groundstrokes are good enough, same for Maria, Justine on the other hand needs to hits more slice and come to the net more, she needs to take pace off and go for the angles and attack the net.
That's about Wimbledon, that's about completing the career GS. I know the media talks about the "golden slam" but I really don't care about it, even though Justine would achieve a career golden slam by winning Wimbledon. I do not think the Olympics can be considered in the mix, best proof are Justine's own words, she played freely at the Olympics but felt the pressure at the US Open, even though she had already won it, even though there are four slams and every year.
Put it this way, if Justine wins Wimbledon next year, she will have won all 4 slams (career gs) and the Olympics (career golden slam) while Serena will have won a career gs but not the golden one, despite that I would still consider Serena's career to be better only because she has won more GS titles (of course unless Justine wins also the Australian Open or RG or both)