People. More diversity of age, gender and ethnicity. We're London based and our clients want to see people that reflect reality. Also as mentioned above "ghost" people with no textures would be very useful during the design phase.
Firstly the latest version - 2.4 - is great. The Asset Library is especially useful. My only issue is the people are somewhat limited. It would be very useful to have people with a greater variance of age and ethnicity. On larger projects there is a need for more variance otherwise we end up with repetition.
We often work in London on large public projects. Our clients are always interested in seeing visuals that represent the reality of the city's diverse population.