Tactical/Strategic Stack Undo: What if the square (or whatever) is already occupied or is not "accesible" anymore? You are only time moving one unit, not the whole board.
Tactical/Strategic Stack TimeTravel: I seem to fail translating this one. So I send Unit A to Limbo for 3 turns. In "real time" that would mean 6 turns for the other units. Why not match the turns in limbo&real time? Effects on the time travelled are not calculated while in Limbo but once they return so no sense to have such difference. And if effects were still calculated while in Limbo, that's not time travel? Are we talking relativity and light speed here? For the sake of sanity, time travelling is better tought as instant in this kind of situations. Needs tweaking but I like it. As long as it doesn't allow to go back in time. Then I hate it.
Strategic Equipment TimeTravel: Ok, this is planar travel and not time travel. This Limbo can have a different time synchrony than the game world, so sending one armour to the nearest city through normal means could take 20 turns but through the Limbo just 5. But it's still planar travel. I do like it tough.
Battle Redo: Err... Someone could say that it already exists and is called "Load Game". Now seriously, no. Imagine this in multiplayer with some sovereigns with some essence reserves, mana and acces to the spell. Imagine to repeat the final battle (a one with similar forces but still final) a few times. Or the AI and if it knows how to properly use it (when a battle is worth repeating for the AI?). And when would it be cast? To be balanced it should be just right after the battle (Yeah, you must repeat your battle of the last turn against a unit that was disbanded this one?). Also, it makes me think of a "Turn Redo" spell, which is something I dislike.
Diplomacy Undo: Ok, this is simple and I like it. Specially is you clicked wrong ("Oh noes! I gave you my Capital by mistake instead of that filthy hamlet in our southern frontier!", I blame random name generation of this). How would it manage to surrender yourself to someone else? Useless for the AI to use?
Limbo: I get the feeling that it has very few to do with time travel and is nothing but an extra plane. I like extra plane. But... I send a unit to Limbo while in battle. Previously to that I had sent some merchants with armours through Limbo. Err... bye bye merchants and armours? If that plane is so tiny that they must fight until there is only one. But if you can have any number of your units there, why not allies? (backstabbing allies doesn't count)
We have two parts here: Time travelling and another plane.
Time travelling is like... uf. If you can redo a battle, why not the last quest you failed? Or as I mentioned, the last turn? Things that involve back in time are bad (altough I liked the Diplomacy redo). Jumping into the future is a quite different beast tough. "Dear Paladin-Captain Leonard, we are low on food so I'm going to send your Paladin unit 20 turns into the future. Hopefully, that will allow us to save some money and food. And maybe when you appear we can afford to keep you guys around. All that for the affordable cost of 100 mana and 5 Essence."
Time effects by themselves could be cool. Some minor spells/enchantments that give "Insight" bonuses to those affected by them (better chance to avoid attacks as you can "see" them coming, or maybe a bonus to attack...). Spells that "insta move" units in the battlefield or "sends them into the future" for a number of turns.
The extra plane is something many of us want. If this plane also has a different synchrony from the game world, it would certainly make it more interesting. This plane would be just a limbo to use as road (or it wouldn't be possible to have such time difference without serious headaches and turn managements). As such, it would be used for allowing faster trade/movement. It wouldn't mean insant travel as unit would take some time to reach theirs destination. Problem is that units sent into the limbo during strategic part of the game, could need food too? Still trying to catch up with the beta so I know that they need money but not sure about food. If they require it, the spell would have a food cost associated to it.
Whichever the desynchrony between Limbo and game world, it could be possible to retrieve units from Limbo during a battle. You could send units into the Limbo with the whole purpose of being able to defend anywhere any time. As long as food doesn't end in their end. Then they would appear in the game world in the same place they were originally (if possible, close to it if not). But while on Limbo, you could summon them to the current battle and hopefully turn the tide of battle in your favor... until the enemy Sovereign summons his dragon from Limbo.
Effects on a unit continue to be calculated while in Limbo. So if the unit has an effect for 5 turns and is sent to Limbo for 10, it'll be debuffed while in Limbo.