One of the biggest markets for small aviation engines which run on something other than 100LL is small drones — running on JP8, Jet A, or Jet A1 would make military logistics far easier and safer. Even road gas would be a reasonable alternative to 100LL, and both diesel/kerosene and gas are easier to find in the bush. Kerosene/diesel are probably better due to risk of water in the fuel at these places, but diesel engines tend to be heavier than gas.

Deltahawk, Thielert, etc are the bigger projects I’ve followed. Getting an actual certified engine is…hard.