Here are some Ideas I would like to add:
Frigates
Arcova Scout Frigate - UNSC Prowler
Cobalt Light Frigate - UNSC Frigate
Protev Colony Ship - UNSC Phoenix-class Colony Ship
Cruisers:
Kodiak Heavy Cruiser - UNSC Destroyer
Percheron Light Carrier - UNSC Carrier
Capital Ships:
Sova Carrier - UNSC Super Carrier
Kol Battle Ship, Akkan Battle Cruiser or Dunov Battle Cruiser - UNSC Marathon-Class Cruiser.
Fighters/Bombers:
TEC Bomber - C709 Longsword-class Interceptor (Bomber Variant)
TEC Fighter - C709 Longsword-class Interceptor (Fighter Variant)
Yes, I know the C709 Longsword-class Interceptor is both a fighter and a bomber, but I think it would work better in Sins if it came in two variants.
Ships without a UNSC counterpart:
Javelis LRM Frigate
Krosov Siege Frigate
Garda Flak Frigate
Hoshiko Robotics Cruiser
Cielo Command Cruiser
Marza Dreadnought
UNSC Ship Based Weapons:
Archer Missiles
Shiva-class nuclear missiles
Mac rounds (used to bombard Planets and in Ship-to-Ship combat)
Pulse Lasers (Prowlers Only)
50mm point defense guns