The ability to fly a helicopter like Valve wanted to- I’m sure the tech is out there now, but you’d have to add new areas to fly it in.
how about drivable vehicles in the multiplayer? would be awesome driving your teammates around as they stand in the back of a truck… or conversely, to fire one of those nifty manually reloadable TOW rockets at a car!
Also, and I’ve mentioned this before, an appearance of the mortars in multiplayer… that could be awesome!
Also, add a few mirrors around black mesa… it’s unfeasable that a major science lab would have no reflective surfaces 
a MP map with the mortar strike would be cool, sort of like the HL2 deathmatch map with the headcrab rocket strike, only with direct control.
exactly… it would be quite entertaining watching players scrambling over the mortar terminal fighting for it’s control!
Another daydream would be to see a remake of Decay in the source engine, perhaps using some of the BlackMesa assets (and eventually remakes of Opposing Force and Blueshift!)
sigh Already. Being. Done.
I think it would be cool to be able to blow chunks out of the Garg’s outer shell and expose the fleshy goodness inside, which you could then shoot.
I’m pretty sure the countless bottles with buffer solutions and other chemical agents that are lining up on the shelves in the labs I’ve been in count as reflective surfaces…
Guess where the second quote is from and I’ll give you a cookie.
The ability to have the expiriment go right and end the game.
Lmfao! That’s priceless!
The QAF?
A gun that can harvest the Xen crystal’s power and shoot out mini-residence casades.
Curious as to what a residence cascade. I envision a row of houses collapsing one after the other like a line of dominoes.
Isn’t there a gun like that in Opposing Force?
I think it’s name is The Displacer or something?
Hmm, I suppose. I guess my dream has come true.
A secret ending where you are pitted against Adrian Shephard in a final climatic battle for main character of HL2 all the while G-Man whispers in your ear “Dance puppetsss Danccce”.
OMG ReSiDeNcE cAsAdEs!
Sorta like the displacer from Opposing Force?
^ King Max said that a few posts before you.
Great minds… accidentally skip over posts while replying. 
Hmm, after watching Naked Gun 2½ with Nordberg building a howitzer out of a handgun, I’d like a silencer on the RPG, a laser-sight on the gluon gun and an underslung grenade-launcher on the glock.