Here is how I'd imagine hard mode (not meant to be super hard, just adds an edge for players who've beaten the game):
-Merchants never refill buy requests or charons, once you fill it they're done buying. Alternatively 45->300 minutes.
-Minimum chance to hit: 10->20%, maximum resist of any type: 95->80%
-Cannot zone transition during combat (Unfortunately would require exceptions to work, so *shrug*). Alternatively, opponents who have ever spotted you upon leaving gain full awareness, 100 initiative bonus and boosted to full health on next encounter.
-10% resists all around added to enemies (before maximum however)
-10 dodge,evasion,fortitude,resolve added to every enemy
-Adrenaline/morphine last 3->2 turns
-Base player MP reduced by 30%
-Player AoE circles (grenades, LoC, etc) one less hex radius
-No more than X number of traps placeable in a room at one time.
Looks like a lot but isn't much for people who don't play severely abuse stuff