Wow – what a great thought. Without going into the realms of the impossible, I would love to focus efforts on creating super-efficient ways of harnessing natural energy, especially solar and storing it it extremely small and efficient batteries. At a practical level, I’d love to banish the need for conventional power grids that require pylons, cables and complex networks of power stations. Being able to generate power at a hyper-local source (I’m talking portable devices like phones and computers that absorb enough energy to power them for days having been left in a slightly sunny area or houses that generate enough power from small cells so they don’t need any grid-fed electricity) would help humanity massively.
Also, anything that replaces annoying amounts of cable in the house with an in-built super efficient battery would be great. I hate trails of cables everywhere!