Wish list for HL2 - Aftermath:
A couple of new weapons: The currently multiplayer only SLAM (tripmine/satchel charge hybrid), the Pulse rifle being restored back to its original E3 state (a funky flare launcher), and possibly the PhysGun mentioned in the article, to weld barricades and armour plating etc. Also that mounted gun the Combine use in the original game looks man portable for a HEV clad scientist.
More defense sections: I can see the Combine really letting loose on the Rebels now, so having a junkyard Helms Deep bunker defense against a swarm of Combine soldiers and APCs, swarms of ManHacks and the occasional Strider would rock.
The Hydra: That big sewer-bourne creepy thing you saw in one of the very early E3 videos (like a large neon-blue snake). Was supposed to be in the original, but I'm not complaining about it's removal too loudly
Gonomes: Since Valve treat the Opposing Force addon as HL canon, it makes sense that these would crop up. Basically headcrab zombies that reach the next stage in their mutation/evolution, these were very fast and immensely powerful zombies that pelted you with their innards like the Quake zombies.
The Gravity Gun: Needs to be toned down again for Aftermath, since it's a uberweapon by the end of the original game. Would be nice if Eli's lab had some more of these for NPC use, as having lots of people throwing rubble at the combine soldiers would be very cool (I can't see them having just one, and then keeping it well away from anything heavy/radioactive that needs moving
)