What about an upgrade to the fusion reactor, the Star Reactor: basically, a miniature star whose energies are contained and harnessed. Maybe more resources and care would need to go towards containment and much more energy to get started, but far more efficient fuel-wise and far more output, or something like that. Of course, any potential containment failure could have some disastrous results...
Or maybe a Mr. Fusion type thing where you can put anything in the reactor as fuel, but it takes a huge volume of stuff and produces very little energy. But maybe it's, like, infinitely scaleable: anything you put in it gets used as fast as possible, as long as you keep supplying it with insane amounts of fuel.
Ah well, just ideas. Really glad this mod is being worked on, in whatever form it takes.