Weapons! I have ideas for weapons!
1) Molotov: Sure, it's been done in L4D and L4D2, but it was originally in the HL2 beta, and there's something to be said about throwing firebombs.
Workings: Basically a cross between a grenade's timer and a SMG grenade's impact explosions, with env_fire liberally added around the blast radius. Fire!
2) Walther PPK/Glock 17: Could be any pistol that's been commonly fitted with a silencer in the past, but these two are my personal favorites. Secondary fire toggles the silencer, if it's equipped to the pistol. Silencer allows stealth killing.
3) Sniper rifle: Either a more mobile version of the Combine Sniper, or a bullet-based one. Less damage than the crossbow, but faster to get to it's target. Possible silenced as well, if it uses bullets.
4) Stunstick: HL2-DM style. Files already in HL2, just need to code it back in for player use.
5) Knife: Same as crowbar, but with slash damage instead of blunt, more damage, and slightly less range. (I think the crowbar uses blunt, anyway)
6) New weapon scripts: I have a weapon soundscript replacement that gives the pistol the close up sound, and the NPC's AR2 the Player's AR2 sound, for consistency. Comes with a slightly improved pistol sound (Both channels of the sound are now identical, so the sound doesn't sound like it's coming from the left of where it is. Let me know if ya want it.
EDIT:
http://developer.valvesoftware.com/wiki/HL2_snippets Ignitable ragdolls would be nice, and if it's possible, could you make ragdolls always spawn as if they were killed by the airboat? (They always seem to turn into expensive ragdolls when you hit them with it, with the ability to get decals on them and collision with more stuff than just the ground)