Some ways that I wanted to expand on the Halo 3 campaign was to add more content than was initially in the game. This includes adding weapons, vehicles, more enemies, etc. I also wanted to give more essence in the story. For example, I wanted to give more of a feeling and believability of the human and elite alliance. Hopefully, this provides enough to give you this feeling. To give you some insight into what has been added, there is now the storm rifle from Halo 4, the spectre from Halo 2, and the shell-grunts from Halo CE.

I also want to give a big thanks to the modders that provided their tags to Nexus to allow us all to use them:

1SDAN - Halo Reach Skirmishers in Halo 3 Using the Halo 3 Jackal Skeleton
A2EX012 - Protector Sentinel Tags
A2EX012 - Halo 3 Shadow (tags)
A2EX012 - MA5K Rifle
A2EX012 - Falcon for Halo 3
A2EX012 - Sentinel Aggressor Captain Tags
AnoleCannoli - Reach and Reach Beta Covenant Crate Tags for Halo 3
AnoleCannoli - Halo 2 Covenant Bipeds - Halo 3 Tag Pack
AnoleCannoli - Halo 2 Anniversary Shade tags for Halo 3
Bunkerjeremy - Halo Wars Blur Elites
Calantorn - H3-ODST Hunters Combi Tags (Mesh Edit New Variants)
Calantorn - H3-ODST Grunts Combi Tags (Mesh Edit New Variants)
Calantorn - H3-ODST Armored_FloodBrutes Tags
Calantorn - H3-H3ODST FloodCombat Vehicle Usage Animations Tags
Calantorn - H3 Colorable DualWielding Flying Brutes Tags (WIP)
DFlogFlag - Woody AI Character tags for Halo 3
FunnyAnyway - Fist of Rukt (Tartarus Hammer)
gnademassica117 - Ultimate Firefight Sandtrap (4.0 Ancient War) Tag Release
jance44vaughn - Sniper Variant tags
Kashiiera - Halo 3 - CE Magnum (Tags)
Killtrocity24 - Halo 2 Capitalship Scenery for H3EK
Krevil - Halo 3 Infinite HUD
MythicJaqui - Tactical Battle Rifle
MythicJaqui - Titanfall Flatline in Halo 3 Weapon Tag
MythicJaqui - Anti-Aircraft Shade Turret
SpartanTanner - Spartan Tanner's Tags
SRR8 - Ghost Pack
TheChunkierBean - Halo 2 Arbiter for the Halo 3 Editing Kit
TheChunkierBean - Halo 4 Master Chief for the Halo 3 Editing Kit
TheChunkierBean - TSCE PBR Weapons for the Halo 3 Editing Kit
TheMindfulMoron - The Mindful Moron's Faster H3 Combat Forms
Vuthakral - Halo 3 Spectre Tags
xM120 - Enhanced ODST - Halo 3
xM120 - Flood ODST - Halo 3
xM120 - M6C Magnum - Halo 3