@FallenTrinityHelium 3 is what I'd suggest.
He3 can be mined in the first few meters of top soil on the Moon, and can likely also be found in Jupiter and Saturn; as a fuel source for no-waste, clean, low-danger nuclear fusion reactors, He3 could be stored in relatively small amounts for a very long run time.
The few companies or nations that make an effort to extract He3 would find their potential for everything so much greater- with He3-powered reactors and the advent of the new Electromagnetic Drive (which provides exponentially higher propulsion than everything else we have now, without exhaust, without fuel, just electricity), then 'capital' ships could truly be a thing. Lasers, railguns, and all sorts of electricity-heavy items would suddenly be very feasible and achievable.
We've got all the ingredients here.
Mining facilities and Helium 3 storage facilities would be among the highest of targets- securing them from the enemy means the difference between an infinite war and one where one side is slowly starved of fuel and resource.
Although, I think this would start off slowly.
What if, say, our players- their pilots and their Titans, both freshly out of boot camp and off the assembly line, are going out for a 'run' outside of their base, where they're stationed until they're assigned to a ship or another base, when it's attacked? Maybe from there, we can go on with the rest of the story, whatever it may be.
And if we're going to be writing a bunch, sure. Casual/advanced, either way works.
Unfortunately, it appears that being realistic and logical turns people off.