I am interested in creating a FPS with a time travel mechanic that would allow players to "travel back in time" over an established period of time (15/30/60 seconds), essentially regaining control over their character in the position they were in at that given time and be able to re-interact with the environment, including killing other players. To do this the game would have to track and repeat player movements after they have already occurred, while also committing states that fall beyond the time buffer (15/30/60 seconds) to the "permanent past".

Hopefully I have explained the general concept well enough. My vision is to implement this game mechanic to change gameplay and strategy within the existing FPS model, not to redefine FPS's. As such my question is whether or not you think something like this could be accomplished through a mod (of say CS:GO, for example), or if this new mechanic fundamentally changes the game so much that creating a new game would be required.

Thanks a lot for you input in advance!

I would like to check what exactly you are looking for a little bit better. If they go back in time, does it mean only the player in that position and all changes that happened since are still the same (aka, I killed someone, but am getting shot at and wish to move backwards, but the kill will still be there).

If that is the case, I would think that it would be simpler to do a mod, as it isn't a huge amount of data to keep track of, beyond player states.

Otherwise, in my opinion, that seems like something that would be more fun to build from the ground up.

You just need a cyclic buffer of undo history for each step in the physics just like when editing a text document but with a physical world. Leaving enemies as dead bodies instead of destroying their object would make it a lot easier to implement with a fixed slot for each item in the level.