In my opinion, the Shivans are the best adversary species in any game ever.
In the first game, Freespace, you encounter a huge giant ship, far larger than anything we humans have. It seems unstoppable and in fact, at the end of the game, you only manage to defeat it by destroying the hyperspace link to Earth.
...then in the second game, you find out that ship was just a scout ship, and they have even bigger more powerful ships in their fleet. But what completely blows your mind, if you play the game, is right when you get a little bit of hope that you can actually destroy one of those gigantic ships, the rest of the fleet shows up, and there are uncountable millions of them.
And if that wasn't cool enough, they show up specifically to destroy a star, sacrificing a lot of their own ships in the process, and you realize that they don't care about those individual ships, and that implies that they never cared about you either. This enemy that you've been fighting for the space of two games is possibly not even conscious of your existence, that's how insignificant you were.
So yeah, if FDev were to envision an enemy like that, it would be amazing. I would love for there to be an enemy that not even the most badass players could defeat. I would love for people to whine, "how is this fun? Why wont you give us weapons to kill them?" and for FDev to reply, "why do you assume that you must be able to defeat everything that shows up in the game?"
I would love for there to be a reason of space where a giant fleet of alien ships is - and it's moving from week to week - and if you go there, they'll swat you like a fly. But, they're not heading to Earth because they don't care about us. We're not important. They're doing whatever they're doing and we can't even necessarily understand it, and we also can't interfere.