Going beyond gaming and mentioning time travel, a united Xeelee universe would not only have a species that can hit you with galaxy-sized weaponry that travels back in time to make sure it doesn't miss, but also their mysterious foes who soundly beat that level of tech somehow, the Photino Birds, who eventually succeed in stopping all supernovas and causing multiple white dwarfs to form instead to make gravity wells they find more comfy. While reading the rest of this, keep in mind that it appears the Photino Birds were so overwhelmingly powerful that they weren't aware the Xeelee even existed.

The Xeelee used time travel to advance their own technology by trillions of years, then went back to the big bang with all of it and started again, and did this multiple times until they reached a technological bottleneck. Their time travel is error free, reliable, cheaply available, common, and something they've weaponized to the point that it's their basic armament. They shoot you with missiles that go back in time and hit you while you were relaxing in bed after school a few years before you even thought of becoming a pilot to fight them. They're also massively FTL, able to move about to any point in the universe pretty much at-will, since this FTL tech is exactly what allows for time travel in the first place.

Humans never managed to do anything but annoy them a little bit, but we do get to add something to the Xeelee arsenal- we found a way to create moving pocket universe outside of the causal universe where time couldn't be manipulated, all for the purpose of avoiding said time-based attacks. They also developed a time traveling tactical computer that analyzed maneuvers, then sent whatever information it processed back to itself in the past, allowing it to outthink superior opponents like the Xeelee. After three thousand years and the deaths of billions of human children, humanity finally succeeded in threatening to destroy the super massive black hole the Xeelee were working on (to save a species that lived there, not kill all humans as we thought) and so they shrugged and wholesale left the galaxy. Humanity in this universe is highly capable compared to a lot of other fictions, yet we could do little more than annoy the Xeelee a little bit. Who themselves could do nothing, not even with reliable time travel on their side, to stop the Photino Birds who never even knew they existed.