Water III: Tsunami.
Although Tsunami fits with the octopus overlords theme, it's pretty disappointing that low level priests have more command over the power of water than an archmage with Water III. It's also frustrating that if you seriously want to fight at sea, you're pretty much railroaded into OO, and water mana is almost irrelevant. I don't think the ability would need any change, but just make the exact same spell that cultists have, available to archmages too.
Passable to impassable will obviously cause problems (making paths computed in earlier turns invalid, without telling the appropriate units that they're invalid), but impassable to passable shouldn't. The pathfinder is designed to handle roads and then railroads, so it can handle decreasing terrain move costs (though it might not do so optimally). Removing a peak just changes the move cost from 'infinity' to '2'.
I don't really see how that is much different... they're both Impassable.Peaks isnt a feature, it's a special type of land.