From my perspective…
The basic idea of difficulty in any game is to turn you from the hunter to the hunted. I don’t really know what I’m getting on about, so I’ll let other games do the thinking for me.
DoomRL: Monsters hit harder and have more HP. On nightmare, they respawn. Wide variety of challenge modes. More monsters = More fun.
Fallout 3: Random first guy you come across takes 20 bullets to take down, and they hit super hard. Because you run out of bullets so quickly, you get creative with how you kill. Explosives and other ordinance are super valuable compared to a regular game. When I find a missile launcher on master difficulty, you bet I take it with me, because a situation where you NEED a missile launcher is right around the corner. In cataclysm, I sort of stockpile my explosives and only bust them out on Fungal Spires. Sure, science labs call for explosives, but turret and secubot placement is so predictable it’s more of an attrition thing. Plus, when you run out of rockets, you just turn off your flashlight and leave. It’s not like they’re guarding anything valuable. I’ve been carrying around a fully charged tazer for two seasons and I haven’t used it once.
More monsters. More unpredictable monsters. Especially high density, variety, and unpredictability of monsters around areas that have the best loot, like Mini-nukes and high end CBMs (the high end CBMs which are most often obtained from shockers in poorly defended cities). Monsters should have abilities that support each other and are designed to break dominant strategies. The smoker is good anti-range. Acid spitters should be more effective at routing players from windows. Dogs keep the pressure on but should be able to grab you and slow you down. Skitterbots should throw flashbangs and manhacks should be hard to shoot. Turrets should be burrowed and invisible until they’re close enough to the player to have a legitimate chance of killing the player, it would give the terranian sonar a use at least. Oh yeah, and hulks tear down walls… and throw players through walls too! And off of cliffs and into the swarm!
Oh, then hardcore mode would double their damage, double their hitpoints, and give them an extra special ability apiece. Even regular zeds should have an ability that helps them fulfill their role as swarming fodder, even if it’s triggered by boomers or shockers. It should be so hard that fortresses get overrun, then when you run to your vehicle, the engine won’t start up for 6 turns while the zombies break all your windshields.