Currently at mid to late game its fairly bland (at least in my opinion) As your expantion has slown down and usually by then you either well on your way to winning. Or you havent done so well and cant catch up to the ai and you will inevitably loose. To make this more than just a boring wait for something to happen why dont you make a mid game event. At the beggining of the game a timer is set. Lets say 150 turns. Once this timer is done, another alien speicies will come and start tearing s*** up basicly. The speicies will start of fairly powerfull and have some major advantidge and weakness, fairly similar to the end game events in stellaris. 


But in stellaris there was only really one way to deal with them, which was to blow them the f**** up. Which, though cool, isnt the most cool of mechanics. Instead why not have a system were you can convince the alien race not to kill your civ, maybe via the infulance preasure bar and something to do with that. Or maybe you could bribe it to go after someone your at war with using a large amount of dust. Or research a special technoligy to stop their ships from leaving a certain constellation entirely making it far easier to kill them. Obvoisly all this options would be hard to pull off as to not make the alien race a complete knock over.


However you could go even further and have ways to delay the timer. SImilar to those pop up events saying "raise the science in the galaxy to so and so" But instead of it giving you 1000 dust and 50 titanium it gives you 10 more turns. Just enough for you to finish another fleet to hold them off. Then it would turn into "Do i want to spend 5 turns on this science building to delay the alien invasion, or do i want to use this production to make another fleet to kill them with" which i think would be really cool, instead of the current mid game being, basicly just "welp this is my 42nd carrier and no ai will ever try and kill me at this point and i will win in about 100 turns so i guess i will make a new game".


But yeah thats basicly it, and it will never make it into the game so i just wasted like 20 minutes typing this all down lol :P oh well.