4/5Fixed and Enhanced Antenna
The Security chest for Halo 3 was originally an accessory of the Security helmet, allowing it to be applied to every chest piece. However, in the most recent update, it was made a standalone chest piece, taking away from that preservation. To restore it to be used with multiple chests AND multiple helmets, Halo 3 could add a new cosmetic option for Accessories; This would allow the player to apply the antenna, flaming head, or katana to any chest piece, without the specific restrictions. The only problem is if they would fit with the Halo Online cosmetics, so as an alternative, each standard Halo 3 chest could have categories and variants like in Halo Reach.
Halo 3 Plague Rat Skull: https://youtu.be/kHzq5w_G1M4
Everyone loves the Halo 3 rat, a skull to replace infection forms with rats would be a hilarious way for 343 to recognize the community's relationship.
ODST Skull: https://www.reddit.com/r/halomods/comments/j6nym8/odst_coop_in_halo_3s_campaign/?utm_medium=android_app&utm_source=share
Since they use identical engines, allowing the player to play as an ODST would be a cool twist on the existing formula (and, if used as mission starting weapons, provide opportunity for the Automag and Silenced SMG in the Campaign)
Heretic Banshee Skin
The skins for UNSC weapons and vehicles have been incredible, but the ones for the Covenant are simply color swaps. It'd be nice if the classic Halo 2 Heretic Banshee was added as a skin option.
Starting Equipment Options
Halo 3 has a lot of options for gametypes but lacks starting options for equipment, so equipment options would be a nice touch for some creative custom game opportunities. Additionally, adding a “random” starting option would be nice for both equipment and armor abilities. Give me more Fiesta insanity.
Halo 3 ODST Armor in Halo 3
Halo 3 has the ODST helmet but lacks any other accessories. It'd be nice if the classic chest, shoulders, and legs were added, as well as Romeo's unique shoulder and each trooper's unique helmet.
Halo Online Elite Armor: https://www.halopedia.org/Armor_permutations_(Halo_Online)#List_of_Elite_permutations
All the Halo Online cosmetics are incredible and I'm grateful that they're being preserved, but it'd be nice if Elites got the same love. Especially with new skin options, parallel to the Spartan techsuit. Additionally, to help fit the new armor in the old art style, some of the textured details could be removed, and first person arms also need to be updated.
Reuse Halo 3's skins for Firefight
Since Halo 3: ODST uses the same models and engine from Halo 3, it'd be nice if skins unlocked for Halo 3 were also unlocked for ODST's firefight.
Expanding ODST Cosmetics
With how restricted Halo 3: ODST's customization is, player's character choices tend to blend. I don't know if it's possible, but it'd be nice if the player could customize their individual armor pieces: left arm, right arm, torso, legs, and helmet. In the Recon Season, the helmeted variant would unlock these pieces, while the helmetless would unlock the character (changing the voice, head underneath the helmet, and fingers). If a player finished the Endure challenge (as this expansion would render the reward for that useless) they would receive Endure nameplates and unique Avatar. If armor expansion isn't possible, granting Visor colors would be a nice alternative, adding the transparent visor from the campaign cutscenes. As Buck and Romeo got more cosmetic options, it'd be nice if Veronica Dare could wear her busted helmet from the campaign too (maybe as an alternative unlock to Endure completion, in honor of the original Vidmaster challenges?)
Firefight Versus: https://www.youtube.com/watch?v=6wIXJRB_mmI
Since Elites are included in ODST's engine, this would grant an opportunity for Firefight Versus in the same way that Halo: Reach offers it; Red team would play as three ODST troopers, while a single Elite would be on blue team with the Elite using the player's Halo 3 cosmetics. The gametype should additionally be functional in matchmaking.
Further Firefight Vehicle and Weapon Options
ODST's firefight options lack many of Halo 3's weapons and vehicles as starting options, it'd be nice if they were all added, alongside a random vehicle option (which would randomize a vehicle per weapon drop).
A common request from the community and a popular mod, adding Flood or individual waves of Flood to firefight would be a blast. If it's not possible to modify individual waves in the future, then it could serve as a general gametype option, where players would select if they want Flood, Covenant, or both alternating (so the flood would clean up fallen Covenant. Gross.)
Vergil didn't have a lot of presence in ODST's gameplay, so adding an option that adds a friendly Engineer to firefight would be fun, where the players only lose on the death of the Engineer. If the formerly mentioned matchmaking design was added, then it'd be put in the same category as Generator Defense.
Cut Firefight Content:https://tcrf.net/Halo:_Reach#Gametypes
A "schism" behavior type, drones, and guta, were all cut from Reach firefight, but function as intended.
Additional Enemy Variants
Halo Reach features Heretic Elites sporting UNSC weapons, but no other covenant who do the same. It'd be nice if they gained those same variants. Zealot Elite squadrons are additionally excluded from Firefight.
BOB Firefight Encounters:https://www.halopedia.org/Seek_and_Destroy
And with Zealot Elite variants, the only ones missing are BOBs. However, unlike the other Elites, they wouldn't be set up in the Firefight wave options. Instead, every Elite would have a 1 in 1365 chance of having its shiny BOB color (golden Rangers, white other variants). Killing it grants an exclusive nameplate, a hefty amount of XP, and the Seek and Destroy medal from Bungie.net.
Lone Wolf in Firefight
There isn't a lot of map variety in firefight, so the Lone Wolf map from the campaign would be a nice touch. However, Firefight maps have safe zones built in for the player to spawn, while Lone Wolf was explicitly designed to be unsafe, so more structures would be needed. Additionally, with its more open spaces, adding a vehicle would compliment its gameplay.
Customize Vehicles in Firefight
Unlike Halo 3: ODST, you cannot customize and choose vehicles in Halo Reach despite the game having more Vehicle maps.
Firefight Voices from ODST
I absolutely adore Dutch, far more than Buck. He might actually be my favorite character in all of Halo, so It'd be cool if the ODST voices were recycled as Firefight voices in Reach.
With the addition of Halo Reach cosmetic categories, it'd be neat if the ODST Chest received its own category and variants with a grenade belt, prosthetic arm, and other options.
Forced Armor Effects per Gametypes
I've noticed people avoid using armor effects outside of Birthday Party with how they affect visibility, so like how colors can be forced, it'd be cool if there were gametype options for forced armor effects, no armor effects at all, or unchanged armor effects. Pestilence in Infection, Legacy on VIP, and None in Ranked games would be incredible.