Maybe use some Mapbase Features for this project.
L4D / Portal 2 style hints.
Dynamic Lighting
Base weapon models changed / reanimated or just reanimated
Some upscaled textures on some props, characters and weapons.
Change muscle car (jalopy) model.
Player can see their own body on death.
Give a more coast side of Half-Life 2 Vibe.
Use some skydomes from other games. Some custom skybox textures for open sea adventures.
More accurate collision on some props ((just for fun))
Try and add interiors to every building along the way, for more exploration.
Alyx Vance has god mode ((i hate when she dies))
Faster airboat and faster vehicle speed.
BACK IN 1998 - Get the crowbar
THE HUNT BEGINS - Get the airboat
SCAVENGE - Loot every island on the open sea areas and loot all interiors
HEAVY WEAPONS - Find the rocket launcher and take down three gun ships
SUBMERGED - Go underwater and find the gunship wreckage
AN EXPERIENCE - Beat the game on normal
TOUGH FUN - Beat the game on balanced
SURVIVALIST - Beat the game on hardcore
MIXED EMOTIONS - Beat the game
CHAIN REACTION - Start a chain reaction of explosives exploding
ALL I WANNA DO IS EAT YOUR BRAINS - Get 100 headshots
CHEST PROBLEMS - Get 50 torso shots
I NEED SOME CRUTCHES - Get 50 leg shots
THEY HAD BARED THE DOOR - Arrive late to Nova Prospect
THE ALYX VANCE - Free Alyx Vance from a combine pod
ABOUT THAT BEER A I OWE YA - Meet Barney on the coast side
HACKER - Get Alyx to hack 150 rollermines
SPRAY AND PRAY - Get fifty automatic gun kills.