OK well let's see...
For ID4Doom I had a shitload of stuff that I made. Let me remember...
* The first boss encounter, the boss walked around the arena and fired at you, etc. However, around the arena there were several sentry turret stations poking out of the ground, and when the boss got near one he would use that to fire at you instead.
* The second boss encounter, the boss was actually the energy core of a alien ship. The energy core shot out friggin hundreds of fireballs at you, etc.
* The third boss encounter, was some big badass on a alien mothership. The boss would run around and fire at you -- but it was different. The boss would fire a shot at you, then instantly teleport away and reappear somewhere else to fire another shot at you, then it would teleport away again, etc etc. The teleporting location was totally random, so it was impossible to tell where it would be next. Plus, it teleported around fast enough that oftentimes it would be out of the way before your projectile weapons hit it.
* The fourth boss was pretty standard, enemy walking around, firing shotgun rapid-fire, shooting homing balls of death which killed you even if they didn't hit you but just came close.
* There were sentry cannons which sat around and shot at you if they saw you.
* There were vent ducts in the walls that hovering drone enemies would come popping out of, and you had to find the switches to close them off.
* There were landmines that blew up when you stepped on them. Also, the landmines were normally invisible since they were buried underground, but if you got a "mine detector" powerup, you could see where they were located -- they would show up as bright orange on the ground.
* There were levels which took place on spaceships, so if you opened an airlock door, you would die instantly, plus you would be sucked out of the door into space.
* One level was right after you blew up the core of the mothership, and the entire level was you running away from the explosions. You ran down corridors, and behind you, the giant fireball would come rushing down the hallways to kill you. This same effect was in one of the levels of Batman Doom, to my great surprise. However, my version (which I never got working totally right) allowed the explosions to turn corners and stuff.
* On a few levels you were piloting a spaceship instead of running around on the ground. In those levels, you would have a spaceship HUD weapon display, and no normal other weapons. Since you didn't know much about the alien vessel you were flying, you couldn't shoot, but you would have to fly around and get past the other alien fighters anyway. Also, those levels had different mechanics so it wasn't just like you were running around -- it actually felt more like you were flying. Also, when you finished the level and went to the next one, you would no longer have the alien pod ship weapon, and you'd have your normal weapons back (though not with the same amount of ammo).
* The alien pod ships you were trying to avoid wouldn't just walk around or fly around like regular Doom monsters -- they ZOOMED around the level like a pod ship would. A weird side effect was that when the pod ships got close to you, they would circlestrafe around you while shooting.
* There was an alien that didn't have any sort of attack. It didn't fire at you or bite you or anything. Instead, it used its mental powers to BLIND you. Your entire viewscreen would go pitch black, and the only way to regain your sight was to kill the alien doing it, or get out of it's line of sight.
* In the fourth act you were on the alien homeworld, so not everyone you ran into were fighters. As you rampaged through the cities of the aliens, you would come upon alien civilians which, when they saw you, would scream and run away. When they got out of sight, they would crouch down and hide. Then, if you ran after them and they saw you again, they would scream and run away again.
* There were teleporters that simply WOULD NOT work until you did something special. Like, you would be able to walk over the teleport pad, but nothing would happen -- you wouldn't be teleported anywhere. You would have to press a switch or do something special for it to work. Also, the same teleporter could send you different places -- like it could send you to one area every time you used it, and then after you pressed a certain switch, it would send you to an entirely different area.
* There were CONTROLLABLE turrets. Like, you got up on this big giant gun, and pressed a switch -- and the gun would fire a rocket. Then you would have to wait a few seconds for it to reload before you could fire it again. Also, another variety of turret, when you pressed the switch, would continually fire plasma out the nozzle of the turret until you backed away from the console. There were going to be levels where you were hopelessly outgunned and had to use turrets to destroy giant streams of aliens coming for you.
* The game had alien monsters which were sort of like fiends from Quake -- they would jump at you from afar, and then slash at you up close.
* One of the levels required you to open a big blast door. You could do it the normal way, which required you to traverse a big level filled with tough monsters that was nearly impossible to beat before finding the switch to open the door, or you could choose an alternate method and blow up every single computer console on the level, which would open the door.
* Certain levels had walks along giant cliffs, and if you fell off a cliff you would die instantly.
That's about all that I can think of. Remember, * ALL OF THIS WAS IN DOOM2.EXE * except for one or two things which required Boom extensions.