Note: This thread may contain spoilers, you are reading this at your own risk.
This is some ideas scrapped from the Halo series.
Halo: Combat Evolved
1. Halo CE was originally going to be a third-person shooter, but Microsoft conviced Bungie to turn it into a first-person on the Xbox.
2. The reason why there is no multiplayer on the Xbox version is because Xbox Live was not made when the game came out.
3. Halo PC was going to have Co-op, but due to re-coding issues it was lost in development.
4. In the level "Keyes", Master Chief was originally going to use the Flamethrower to rip out Keyes' skull, but the weapon was removed.
5. The creature "Blind Wolf" did not make it into the game because it was too difficult too control.
6. The "Gravity Wrench" was cut from this game.
7. There was originally a machete in the game. Master Chief's original helmet was the ODST helmet.
Halo 2
1. The "juggernaut" was a huge Flood creature capable of killing any enemy with one hit. Due to this reason, it was removed.
2. The Covenant species "Drinol" was removed from the final game.
3. "Alien Trooper" was removed from Halo 2 according to Halo 2 Collector's Pack.
4. The "Doozy" was removed from Halo CE and Halo 2.
5. The "UNSC Strike Fighter" was never added.
6. "Shielded Carrier Forms" are basically carrier forms with enery shields, they were cut probably because it would be too hard to kill.
7. "The Special Purpose Sniper" was removed from Halo 2.
8. 3 levels was cut. "Covenant Ship, "Forerunner Tank" and "Forerunnership"
9. The "Kill Bug" was cut from Halo 2. "Stalker" was also cut from the game as well. So was "Alien Trooper"
Halo 3
1. 3 Pure Froms was removed; "Infector Form" "Transport Form" and "Stealth Form"
2. The level "Guardian Forest was cut.
3. Firefight was going to be Halo 3, but it was removed due to time restraints.
4. Hunters was going to be fighting alongside with the Master Chief, but it was put on the Covenant team to prevent confusion with Loyalist and Seperatists.
5. In "Cortana" the room where Cortana is located is meant to be a remake of "High Charity".
6. "Cortana" was originally split into two levels.
7. "The Covenant" is the last apparance of the Covenant.
8. "The Covenant" is also the only level where Flood is your allies.
Halo 3: ODST
N/A
Halo: Reach
1. Several ideas was formed on the "Guta" which were all cut.
2. The Guta was originally going to be in several missions besides Nightfall, but due to resource constraints Nightfall is their only appearance.
3. Space Animals was cut to prevent players from thinking it's okay to shoot real-life animals.
4. A Scarab level was scrapped from Halo: Reach.
Miscellaneous
1. Halo: Combat Evolved was originally meant to be the only Halo Game, but due to it's popularity Halo 2 was made.
2. Bungie originally came up with an idea for "Halo 4" before making "Halo: Reach".
3. Microsoft has verified that there will not be a Halo: CE remake (unconfirmed)
Got any more scrapped ideas? Post it here.
[Edited on 01.23.2011 10:45 AM PST]