But what options are there for fission wastes once we develop a virtually costless energy supply?
Exactly what they are now. Encase them in a bunch of concrete and other materials. Go out to sea. Find a subduction zone. Dump away.
In the newest reactors, much of the waste is recycled and put back in the reactor.
Also, given enough energy, liquid hydrogen is the way to go for liquid fuel. You can use it in combustion engines, or even produce electricity in power cells. The only waste is heat and water.