Besides a complete cooking like frying, baking, BBQ or smoking is there any other way to prepare freshwater fish that would render it safe to eat in a sushi type application?
In many** parts of the world, fresh water fish, and small species coastal fish are perfectly fine to eat raw without further processing
Check with your local government food standards department on the water and fish quality where you live
** By land area, not by population. In most heavily populated areas you cannot eat fresh fish, or coastal fish raw
I wouldn't recommend trying, really. It's safest to just cook the fish.
Curing with lots of salt (as in gravlax) or acid (as in ceviche) would take care of most bacteria. Freezing at a very low temperature (0°F) for 24 hours can kill most parasites, but home freezers cannot get cold enough. If the fish was previously (and industrially) frozen, you might be able to use it in a ceviche preparation. Just don't send me the hospital bill.