Rather than attempting to model and animate a more realistic skirt, I decided instead to outfit Luthien in pants. I still wanted to keep the deep blue color scheme for her clothes, and I still wanted to keep her outfit rather simple, and this is what I came up with for her:
A little more detailed than the original, but still quite simple.
The simple outfit is not just to keep the poly count low (although that helps a lot). I want to keep her outfit simple because I think elves are usually depicted as a highly ornamented, luxury-rich, and artistic bunch, and I wanted to go in another direction with them, making them less attached to the physical world and living more spartan lives. More about study, research, and philosophy and less about plinky lute music and elaborate gold earrings.
I also tweaked her run cycle now so her legs come closer together and her arms cross over her chest as she runs. Hopefully, it looks a little better.