I've done 35mm slides from 35mm negatives, using Kodak 2302 (B&W movie film for release prints) developed to the highest contrast I could (in stock Dektol).

Given that I have a 6x6 projector, I have thought about doing something similar with 120 negatives.

My first try would be using available Efke Print Film (it's for sale at Freestyle). Contact print under a glass sheet, then develop by inspection in stock Dektol.

There are lith films (APHS) available as well. I've done an 8x10 transparency by projecting a negative onto such a film, then developed. I suppose contact printing could also work.

The first two options are rather cheap; after that, you could go with more expensive products such as ortho films: Rollei Ortho 25 in 120 is available, and Ilford Ortho+, as already mentioned, is available in 4x5 and above.

But the key is: finding a film that will get to a very high contrast. If your average film has a gamma around 0.6, and you need a final slide gamma of 1.5, then by a rule of third, your positive film should be able to develop all the way to an inherent gamma of 1.5 / 0.6 = 2.5, which is extremely high.