All old weapons, i like limited range weapons that do damage over time alot, so imho they could add something like a Shaft/Lightninggun or FLammethrower.
The Shotgun should be able to hit things that are farther away than a few yards.
Big Monsters, like the Cyberdemon and Spidermastermind.
A Boss you have to actually fight, no fancy game mechanics or scripts, some big thing that throws everything it has at you, you gotta outmaneuver and outshoot it, the end.
No Sniperrifle or anything close to one.
Scripts and Videos: Yes, but not too much, you should be able to speedrun the thing.
PLEASE keep Keycards, PDAs as Keys, its one of the few things i hate about half-life, every door has to opened via a switch.