Remake Quake's.
Offhand hook (tied to a pickup item), immediate pull (accelerating/decelerating), swing, dangling, mwheel reeling, safety dropping (impact damage...) and only allowed on mapper-placed grapple points (think Tomb Raider). On death, you lose the grapple and it can be found in the backpack.
A single grapple point allows several moves, combined with air control and acceleration, you can trickgrapple around two corners and onto a walkway etc while reaching speeds of over 1000 ups. A map will typically have between one and six grapple points which is totally enough. Singleplayer will have more than DM (for example DM6RQ only has a single grapple point near the RA which is enough to add several new moves).
Due to the grapple point system, it can and is being used in singleplayer for all kinds of weird stuff. Grappling monsters is fun, too, especially underwater so it lets you close the distance quickly (before the bastards have a chance to drown or escape). In a game where half your weapons don't work while submerged, this can come in handy.
The last word in grappling I hope. And the final argument to get a multi-button mouse for singleplayer.
