Think about this. Within 40 years the dream of fusion power will be a reality, but what's after that? No really what will we be developing after we've harnessed the Sun's power source. Think out of the box for this one.

A few examples I thought of:
-Singularity Reactor
-Zero Point Energy Extractor
-Antimatter Reactor
-Virtual Particle Extractor
-Dark Energy Extractor