The original Insurgency had a Hardcore and Elite mode for Coop. I'd like to see Hardcore atleast return.

The way Hardcore worked in the original insurgency was that insurgents were paramilitary. I'd like this to be sorta represented in Sandstorm. AI insurgents would regularly wear camo, balaclavas, helmets, and regularly wield powerful rifles, perhaps even some Security-exclusive weapons. Overrall, make them look more organised. For Security, you could have them all wear camo suited to the map, as well as just overrall making them look more uniform like an actual military (remember, this is all for Hardcore only).
I think another addition to hardcore would be to make the enemy call in a LOT more fire support. Like, use it all up by the end of a round. I think you could also have more counter-attacks, and add some unique vehicles specifically for them, like heavily armoured vehicles, maybe even tanks.

Elite: This was meant to have more realistic roles and loadouts, and be more restrictive. This could potentially remove fire support from this difficulty, or lower the amount of fire support (e.g. 3 explosive and smoke artillery/mortars, 1 gas mortar for insurgents and 1 A-10 for Security). For example you could have classes be organised like this:
Rifleman: M4s and M16s for security, AKMs and AK-74s for insurgents
Breacher: Shotgun and SMG
Demolitions: Same as riflemen but with mounted launchers, plus single shot rocket launchers/recoilless rifles
Gunner: M249 or PKM only
Advisor: L85 and MK17 for Security, FAL and SKS for Insurgents
Marksman: MK14 and M24 for Security, SVD and mosin for Insurgents
If you did include fire support:
Commander: Mark 18 for Security, AKS-74U for insurgents
Observer: G3 for Security, FAL for insurgents
Alternatively, you could use the Firefight classes for this version.