The late game crisis is such a good idea to solve this problem.
At that point you're strong enough that you can beat all the other empires but then an extra-dimensional invasion happens that's just so much stronger than anything else you've encountered and it works really well to spice up the late game and make everything relevant again.
At that point you're strong enough that you can beat all the other empires but then an extra-dimensional invasion happens that's just so much stronger than anything else you've encountered and it works really well to spice up the late game and make everything relevant again.