Impossible unlocks once you've completed the campaign, mostly because it's a much bigger jump this time. It's not the flat 10% health and speed from the older games anymore, the scaling varies per terrain and enemies get their own upgrades on top of it, so shamans heal faster, some enemies pick up extra armor or dodge, and so on.
Veteran and the different game modes are a serious challenge in its own right. And by the time Impossible opens up, you'll have your heroes and spells where they need to be for it, which matters more than it used to.
We hope you give it a go and let us know what you think!