In order to get more dynamic range from high-ish output pickups, is it possible to implement a new effect that is the opposite of a compressor ?
It would make the output more sensitive to touch by “amplifying” the gain differences in the guitar signal.
Sounds like a gain block which already exists 
You mean an expander
The inverse of a compressor is an expander like @Niksounds mentioned, but an expander is actually just a gate. If you put in a “Utility Gate” block and increase the “Range” knob to something more reasonable, you have the exact effect you are asking for.
That said, I don’t think this will sound the way you’re expecting it to on a guitar signal.