'Fuel' or 'Energy'.  It can be gathered in a variety of ways - nuclear power (possibly need a new building: Nuclear Fusion Plant), solar collection (possibly a new unit aka Solar Collector), planet mining (only possible on gas giant skin planets, new building aka (Gas Extraction Facility)).  There might be other ways but I haven't thought of them yet.

It could be used to build fleet units (e.g. bombers, figs, transports and e-ships) and then cost a smaller amount to move them places.  With e-ships the fuel cost for moving might be a little higher on a unit-by-unit basis since they're supposed to be one-shot.

I was thinking there could be a limit on how many fuel types an empire could use; perhaps only one or two.  You could pick what fuel/energy to use at the start of a round (or whenever you random in) by going to the Research tab and selecting it there.

Oh and there would be a new research option called Fuel (or Energy) Efficiency.  Higher research in this area would obviously allow players to create and move units using a lower amount of fuel/energy than is default.