LogFAQs > #902060217

Halo 2's ending was cut short due to time constraints, leading to the infamous cliffhanger. The final level was originally going to involve Master Chief driving a Ghost through the Forerunner Dreadnought la the Warthog run at the end of the first game, with the final cutscenes being those of the opening Halo 3 cinematic where he crashes to earth and the Halo 3 reveal trailer where he witnesses the portal to the Ark.

Halo 3 was also going to feature some different levels and fights that didn't make the cut:

The multiplayer map Guardian was based on a cut campaign level where you travel through jungle catwalks on the Ark and encounter "guardian sentinel" minibosses (you can see a hologram of one on Epitaph).

At the end of Cortana you were originally going to fight the Gravemind himself with a downed Scarab trying to get its gun operational while you dodge his tentacles. Cutting this boss fight effectively made it so you never saw the Gravemind directly in Halo 3 (something I felt was rather unfitting).

Between Cortana and Halo there was a level where you hitch a ride on a giant "builder sentinel" to the newly made halo ring instead of flying there in a Pelican cutscene. These sentinels are what strip the planet at the center of the Ark for resources to build halos.

Bungie was also legitimately considering a Halo 4 as their final Halo game (hence the Legendary ending) but decided on Reach instead.