If there's ever a Halo collab, what would you expect? They probably won't give us the assault rifle due to a lack of sights. I'm going to bet on a silenced SMG, the shotgun, and the battle rifle.
Well, I see it going one of two ways (theoretically), either maximum nostalgia with the ODST stuff or trying to plug Infinite or whatever the current product is at the time. I imagine given the "where is the line" post that ODSTs, Marines, and maybe S-IIIs/IVs would fit the power level of Helldivers better.
M7S SMG, maybe the normal M7 (might as well if you're gonna model it), Magnun, silenced Magnum, BR-55, the Spanker, and the chaingun emplacement as a heavy MG hip-fired strategem would be really fun. They could probably add any of the MA-5 ARs, I think some of the Infinite ones have holographic "iron sights." Plasma grenades would make my day.
ETA:
Assuming it's a 3-Page Warbond format, maybe like
PG. 1: UNSC Army Trooper
Halo Reach Trooper Armor (Siege-Ready Medium)
MA-37 Assault Rifle
M-392 DMR
(Spnkr if we're lucky)
Civilian Warthog FRV Skin
Fall of Reach Memorial Cape
PG. 2: ODST Pt. 1
HALO 2 ODST Armor (Medium)
BR-55 Battle Rifle
M7 SMG
SRS-99AM Sniper Rifle Strategem (or a cosmetic reskin of the AMR)
Plasma Grenade
"HELLJUMPER" Player Title
PG. 3: ODST Pt. 2
Halo ODST Armor (Medium, maybe Buck or Dare variants? In an ideal world both lol).
M7S Silenced SMG
M6S Silenced Magnum
AIE-486H Chaingun Strategem
"Warthog" FRV Skin
"Rookie" Player Title
But we're talking like a $40 crossover easily with that wishlist haha.