Hardened Seven Years War => Reward: Little Rocket Man => All your mortars are instantly loaded and ready to strike (Seoul).
Hardened Thirty Years War => Reward: Medical Malpractice => The time it takes to upgrade hospitals is cut in half.
Hardened Hex Earth => Reward: Buff and Polish => The time it takes to upgrade an artifact is reduced by 25%.

Bonus:
Hardened Tutorial => Reward: Smog Free Zone => Your fog buster artifact will reveal 60x more territories.

Add Set Bonuses:
Complete 5 Hardened Stages to unlock => Reward: Alien Technology => Artifacts produce 10x the results but their cooldown time is doubled (does not work on Idle Battles).

Basically, reduce the time of stuff that is currently immutable and give an incentive to people that super ascend to do stages other than Hardened Ursa:Luna/Copper Creek/Scandinavia.

Edit: I wanted a bonus that add +1 to mortar range and makes all targets at a radius 1 or 2 100% hit chance but all the mortar stuff should probably be done to the base game since they are awful.

Edited 3/27/2023 00:11:23

@Math
I would actually like to see a Hardened Hex Earth, that could be special; and could give one of these rewards

we need a Hardened Tutorial, it will be the end boss of idle

Hardend Tutorial should update the normal Tutorial to include a tech tree that gets explained and add crafters and markets with a detailed explanation. Hardend Tutorial should only unlock after ascencion.