As I see it, restarting the game process from a distant time point in the past is too drastic a change.
But the idea of "time warfare" itself is quite interesting. In science fiction it is played out in different ways, for example the Terminator movies come to mind.
How could this happen in an X-Com game, based on the idea of the enemy base problem?
Suppose research shows that there is an enemy base that is so well fortified that X-Com is currently powerless to capture or destroy it. And there is information that the existence of this base threatens to defeat the war. For example, the base will begin to expand rapidly, taking over more and more territory. As far as I remember, the enemy bases have such a possibility - to send UFOs, so that they founded more and more new bases, saw a screenshot of this.
That is, you need to somehow make the first enemy base is invulnerable to X-Com attack.
Then the only option would be a past operation - sending a special group to destroy a base when it was just founded and no different than a normal base. Such a journey would only be possible with the use of a huge amount of Eleurium-115, which is a unique resource.
This would require launching a combat mission based on the results of the research and only if the necessary resources are available. Externally, such a launch should work similarly to a flight to Mars. Instead of specifying a point on the map, there should be a separate button to send a special intertemporal jumper.
If the mission fails, the transport and everyone on it is lost, and the enemy base remains in its previous location. If successful, the enemy base disappears from the map as having been destroyed long ago, and the plane returns with the surviving soldiers... who must now explain who they are and where they came from to management. After all, no one sent them anywhere. Destroying the base in the past made the mission unnecessary.
Anyway, that's the basic idea. Not a restart of the game process, but a local operation in the past to change the situation in the present. I have no idea how feasible this is. And whether it's worth doing.