My last one didn't quite have what I wanted in it, so here are the rest of them! HARD MODE replaces all regular infected with nothing but Riot Zombies, while The Lootening replaces all of them with Fallen Survivors* with loads of stuff to drop upon death. While loads of health-dropping infected sounds easy, keep in mind they don't chase pipe bombs, can't be set aflame, and have a few times more health than common infected.
*This vpk is incompatible with any others that may change the gamemode.txt file; if any others are present, only 'one' Fallen Survivor will appear at a time. Otherwise you'll see dozens of them at once.