I suspect it's contrary to your objective of having attractive tile on which to walk, but there are a number of grip-enhancing paints on the market. Many are described as driveway paint with grip or driveway paint with grit. 

Other links suggest adding job-specific grit to paint and from there one could extrapolate that the same grit could be added to a clear surface treatment.

Grit embedded tape is also on the market, far less attractive than a painted surface (opinion) and subject to unsightly wear.

A final resort would be to hire a portable sandblaster vendor or device and remove the slippery glaze of the tile. The coloration would likely remain but become subdued.