I do not think such a spell would work good with multiplayer, because who wants to force other players to replay a round. Nor do I think it would add much to the SP game, either.
That said.....
Why all the hate against saving/ reloading outside a non-ladder, non-multiplayer game? Sometimes, that is the best way to learn from mistakes so when "showtime" is on and you are in a clan battle or doing a SP map contest at a fansite, you will be better at tactics.
I will give you two recent examples.
I was playing an Age of Wonders: SM game versus 7 AI opponents. I made a fatal flaw of getting too focused on eliminating a opponent I saw fairly early while neglecting exploring the map and making sure I had good garrisons where my wizard was and a critical city. I threw everything I had at that opponent. Well, what do you know? Two other wizards came in with 2 heroes and a few troops from nowhere and eliminated my fledgling empire. Restart again, and now I learn the lesson of garrisons and scouting and only going all out when I have scouted and am sure I am safe versus blindsides. I didnt want to start over, because I wanted to try a rush tactic.
I was playing Civ 4: BtS on a huge map with 16 opponents. I was in the middle of major micro and got a bit too focused on one thing. I didn't pay attention to Pericles massing troops up or the fact that I was friendly with my other nieghbor, Isabella and did not have a defensive pact. When war happened, I was totally screwed and had no tech to offer Izzy to help me. Go back one turn, and I learn the lesson of being mindful of my situation. Defensive pact with Izzy keeps Pericles off my lawn. I go on to space race victory.
I would never do this in a serious game or tourney. But if I am trying strats with my own game and time. By all means, let me! And let Stardock put a # times saved and actual time of each save header on my saved game so tourney runners can check my honesty if need be.