Just a list of units (I would like) to add.
M145D Rhino - A self-propelled artillery piece seeing renewed interest in the post-war period. Optional upgrade, 'Zeus' plasma cannon
HRUNTING/YGGDRASIL Mark I ADS or Prototype Suit. May function better as a 1-use super unit that is expended after one battle, but is very powerful and expesnive
HRUNTING/YGGDRASIL Mark II [D] armor system- AKA the big mantis from Adrift. May function well as the UNSC counterpart to a Scarab or War Sphinx
Kestrel- Counterpart to the Ghost
M510 Mammoth- obvious vehicle is abovious
M312 and M313 Elephants- Halo 3 and Halo Wars variants, respectively.
Hellbringers- flamethrower squads, maybe an upgrade of marine?
M850 Grizzly tank- because Grizzlys are awesome
Cyclops- the Halo Waypoint page on this gives many variants and potential upgrades for the cyclops
Spartan-III- SPI armour could make them have low health but harder to hit. Headhunters upgrade?
M247[M] Gungoose upgrade for the mongoose!