Baby or massage oil coupled with a brush (electrical or not) have devastating results. To such an extent in fact that my girlfriend sometimes absolutely forbids me use that combo on her on those days when she feels she won't be able to take it. Back in my student years, I tied up an already ticklish a woman on a crappy bed before doing that to her... and it completely fell apart after something like ten minutes because she was struggling so wildly. Okay, it was not the best quality of furniture to begin with, but talk about enhancing ticklishness!
Having walked around for some time in closed shoes makes her feet really feel soft. You can make her scream with just a few scratches after a shopping spree or following the after-work commute back home. Also, my girlfriend has a stone-like thing that she uses on her own feet once or twice a week, usually after taking a shower. I am not sure what it is exactly (girls and their beauty secrets ^^), but it looks like pumice-stone to me. After she has used that, the soles of her feet feel super soft, and I do mean extra SILK-soft. In these moments, so much as blowing on her toes makes her giggle.
Finally, if you are into this kind of things, nettles "burns" make feet more sensitive after the pain has settled. Some pharmacies, especially traditional Chinese ones, offer diluted nettles venom solutions for footbaths which have a medical purpose... but make for a great "torture" foreplay as well 😈, much better than itching powder at any rate. Once again, if that is your thing.