Would it be possible to add damage types to the weapons (unless there already is such a thing), and add different DT and DR depending on type of damage to the armour? So some armour would be more effective against different types of damage, and weak against one or more of the other damage types. Like in Fallout 1 & 2, and not just a flat DT across the board as it appears to be in FNV.

I believe the Fallout 1&2 games had the following damage types: Plasma, Normal, Fire, Explosives, Laser, and Electric. Which would of course be changed to match Fallout NV world better.

IIRC the armors really didn't differ significantly in their protectiveness against different damage sources, the most difference was +-1 or 2 DT and 10% DR. Only exception was lasers (but not plasma) got totally shafted by high level armors, and lasers certainly aren't in need of a nerf as things stand in New Vegas.

Would be cool to move all armor and weapons back to the Fallout 1/2 system in general. As it is there is too much HP and damage inflation, and Damage Threshold doesn't nearly do enough. Power armor is supposed to make you [censored] invincible.

Not easily possible, as armour only has one DT value. You can apply multiple different types of resist (a la FO3's fire resist/energy resist) using object effects, but beyond that you'd need some messy scripts and perks added to every armour & actor.