If we can't make a breakthrough in fusion before we run out…

If we can't make a breakthrough in fusion before we run out of cheap oil, then the best alternative we have almost already ready, next to fission, is algae-based biofuels. So far they have been too expensive to produce, but they essentially only consume space and sunlight, and can be manufactured into a form of energy as dense as fuel, which has proven hard to beat. Other potential breakthroughs are probably in solar-based hydrogen production or uranium microreactors.