A terran alternative could be interesting but.. the aliens are the high tech species in this game, not us. If something were easier to produce and equally as good, why would they carry plasma? If it is superior to plasma, why can we come up with it in less than a year from 1999 ordinary human tech/science, when they presumably had much longer. We're good at making weapons, yes, but the aliens are an alliance of planet conquering species. They must be pretty good too!
The only thing I can see is something that's wasteful. Like a 1 elerium per shot weapon that is outright better than plasma. We're desperate because if we lose here, we don't exist anymore as a species (or just as slaves), so we don't care about the waste, the pollution, etc. The aliens can afford to value efficiency more than we do, since winning is probably less important for them and they want to win a usable planet. If we lose, you bet I want the Earth to be a worthless wasteland for them to conquer.
Then there is the self-consistency problem of a weapon using depleted Elerium being as strong as one using active Elerium. That's a bit weird. The depleted Elerium needs some kind of downside. Unless we compare depleted Elerium to the blaster launcher, which is active Elerium based, instead of plasma. But outfitting every soldier with a (weaker, terran) version of the blaster launcher just destroys game balance.
And that brings us to the last point: balance. It's not about one-upping the aliens, getting the next
BFG and then stomping all over them. It's about the game being challenging so it is enjoyable. So if we create a next tier weapon/affordable blaster launcher, there needs to be something for aliens too.