I think the strongest priorities are 1) new clothing meshes for long pants and skirts, and 2) solo posing and masturbation. These are less a matter of my personal preferences and more a matter of covering the basics to make sexsim competitive with other sims. Once those things are in, you've got the fundamentals covered and can expand from there.
Other things I'd personally like to see, in no particular order
more shoe tpyes (more high heel types, flats, cowboy boots),
more hairstyles (pony tails, pig tails seem basic),
additional locations,
anal.