Hmmm... interesting thought. How to improve manual post hole diggers?
Ergonomic handgrips on the handles for better impact resistance.
Serrated teeth on the shovel end to chew through roots and clay.
Resistance springs between the handles to ensure a better hold on the dirt you are removing. (It tends to drop out if you release pressure while pulling up.)
Other than that the best improvement i've ever seen with them is the fiberglass handle. Weathers so much better than the old wooden handles.
I hope this gets ya an A+, If not make ya a million.
"Quality isn't expensive, It's Priceless!" By unknown.
South Carolina contractor