Power systems, particularly for small scale power generation and vehicles have all revolved around old technologies like internal combustion, external combustion engines and recently around pie in the sky fuel cells.
The direction I feel power system development needs to go is towards muscle power. Muscle power doesn't mean human or animal power but artificial muscle (which mimics natural muscle) and/or the use of lab-cultured and bioengineered muscle to power machines. The reason natural muscle would have to be bioengineered is because it would have to withstand extremes of temperature, moisture and activity.
Such power systems must get their energy from organic matter: "nutrients" (inherently so for the bioengineered muscle) so they would naturally be both sustainable and environmentally clean. They would also have completely silent operation.
In fact, it would be splendid if individuals cooperatively developed such power systems publicly and in a way that just about anyone could make their own. If goats can be made to produce spider webs in their milk then certainly such muscle can be developed through genetic engineering.
In practical terms, the simplest version would be a sealed container with a shaft having at least two eccentrics (like a V-Twin crankshaft) held in bearings on both ends. The "muscles" would have one end connected to the shaft's respective eccentrics with rings containing their own bearings and their other ends anchored perpendicular to the shaft (like the connecting rods in the V-Twin) to the container via similar rings.
Besides this "Muscle Motor" of course, a tank for it's energy source, a pump to convey it, perhaps a tank to collect the used fluid and the respective plumbing would be needed.
Needless to say, the applications for such technologies are as diverse as those of electric motors and combustion engines, not to mention hydraulics and pneumatics.