It would be nice to have the ability to script actors and making them walk or enter cars, putting an arrow above a car, being able to play a cutscene for a player and special-actor skins.

Edit: some more:
sphere.Pos fix
Walking styles (0245: set_actor $MISTY_CHAR walk_style_to 15 )
Arrow above players (0187: $1546 = create_marker_above_actor $MISTY_CHAR )
play a sound on a specific location // something like sound.PlayAt( player, x, y, z )
New dialog messages: http://screensnapr.com/e/X1RRUo.png ( 03E5: text_box 'HELP12'  // Walk into the center of the blue marker to trigger a mission. )
ability to move objects, like in SCM modding:
move_object $MY_OBJECT_HANDLE to 0.0 0.0 100.0 speed 0.0 0.0 0.1 flag 1
034E: unknown_move_object $AIRPORT_DOOR2 to -770.375 -602.875 11.8125 unknown_angle 0.0625 0.0625 0.0625 0

Also, I've found some functions in the GTA III SCM:

Setting weapon accuracy (not sure if this can be set already in weapon.dat)
02E2: set_actor $ASUKA2_MAFIA4 weapon_accuracy_to 70
Set infinite run for player
0330: set_player $PLAYER_CHAR infinite_run_to 1

Set car immunites (the code below makes the car bulletproof)
02AC: set_car $3310 immunities 1 0 0 0 0
params: bool bulletproof, bool flameproof, bool explosion proof, bool dent proof, bool exploded texture? (80% sure theyre correct)

And some from the GTA VC SCM (not sure if they can be used in GTA III):
Setting car animations instead of player animations (This is used in ie. the Admiral in the first mission of VC)
0477: set_car $2469 animation 1 500 ms
Making car tires immune
set_car $2296 tires_vulnerable 0

Edit 2:
add bool vehicle.Alarm (setting and returning)
Cranes! :
01EE: activate_crane 1570.25 -675.3749 1638.6875 -687.0624 1647.875 -700.0624 1571.0625 -696.4999 16.0 0.0
01EF: deactivate_crane 1570.25 -675.3749
0368: create_ev_crane 1570.25 -675.3749 1565.6875 -686.4999 1576.75 -706.5624 1639.875 -696.6874 26.0 0.0
02FB: create_crusher_crane 1119.75 51.75 1135.75 56.0625 1149.75 46.25 1143.0 59.875 5.0 180.0
03EC:   ev_crane_collected_all_cars
80B0:   not car $CAR_EV_CRANE 0 1668.5625 -685.6874 1548.0625 -745.4999
83A0:   not car $1483 picked_up_by_crane 1120.0 46.0

AttachObjectToVehicle, AttachObjectToPlayer, also this would be great:

http://youtu.be/xfgMn_Q0K-E

Merry Xmas and a happy new year to all of you!