I saw a Tomb Guardian pick up a Proximity Grenade and throw it back at my people. I was both surprised and impressed.
Raiders should be able to:
1. pick up weapons and fire them at your team
2. Pick up grenades or stones and throw them at you
3. STEAL YOUR RIDE!!!!
They board your Skyranger, etc.. kill the pilot/guards and steal it!!
Now you have to radio in to base so they can send a rescue ship. Or else your Team is stranded until you get a new ship built!
4. Then OpenXCOM regularly - weekly, etc.. - calculates how many wild beasts / monsters attack you, evaluate your ammo and thus gradually your stranded team can & will perish.