the game already has a nice animation system, it shouldent be hard to animate for a team of profesionals.
its a realy simple mechanic, aim at ledge, press the use button and then move forward to climb.
to add lots of depth, the player could possibly grab a handhold, move up to grab another handhold, and climb all the way up. entire dungeons could be built from this mechanic!
for the player, this would be- aim for object, grab object,aim for and grab next object. use the arrow keys to control your body whilst holding it, you could climb up, sideways, down and over ledges, you should also be able to jump from a wall onto another wall, or perhaps even risk a jumping grab.
2: make the duck/sneak key interesting!
sprinting and then holding duck down should put the player into a slide tackle- you should be able to slide down big slopes like this on mountains, or perhaps slide across lakes- but the main thing to do is to dodge things and quickly escape through low objects
jumping from a great hight and ducking at the last moment should put the player into a roll, if the player is thrown by a spell or a creature then the player should also be able to roll to land with less damage and faster getting up.
3: a jump spell...
4: the ability to run a little on walls- not a lot like some high fantasy games, but a little. but this should be nigh impossible in heavy Armour and should require a high acrobatics skill
5: a wall walking spell
play the game aliens vs predator ( the one from 1999 or avp2 ) and you will see what i mean , it would be fantastic for stealth players