Would it be possible in theory to develop a material with very slow velocity factor to build dipoles immersed in a slow velocity factor gas in order to make much smaller HF antennas?
Instead of using a high permittivity medium, you can load your antenna with lumped elements. You may have seen whip antennas on vehicles that have a large helical coil at their base.
Be aware that electrically small antennas have two drawbacks: first, they don’t couple as efficiently to EM waves as a full-sized antenna, and secondly, they generally have smaller bandwidth.