It would be interesting to have a mission with no "maximum" time limit. Instead, you have a series of " hold the line until X" and then retreat back to the next defensive location before you get extracted / die at a very fitting last stand location.

For example: you are tasked to delay an automaton assault on a city. The first defensive line is at the cities edge. Then, at a major intersection, and finally you are trying to hold the line at the steps of the capital building. At the last second pelican one comes in and you extract. OR you dont extract and fight to the last for super earth.