Or just a Stargate type of game. Always wished for a game about showing up in various past civilizations with modern tech and knowledge, even if it was just a for sh*** n' giggles mod, haha
"Aaaaand I just destroyed the entire Confederate army with one AC-130." "Aaaaaand I just held off the barbarian horde with an M240."
What an intriguing idea for a game. The first thing that comes to mind is when jumping back and forth between times you might see the results of your actions in the past. Play a level in the past, jump forward and see a completely different world than the last time you visited.