I would tie it in with natural expectations. An easy climb sees Nate whizz up it. And impossible climb sees him fail to start. A moderate difficult would slow the assent with searching limbs - you couldn't use a "quick vertical getaway" to climb yourself out of trouble. A hard climb would have a risk of slipping, with consequences depending on how far you fall, forcing the player to role-play the character. Would they really attempt to climb that shere cliff-face when they keep slipping all the time? Just because you can climb, doesn't mean you should, and the player should be guided into intelligent decision making IMO.
I see. How much assistance should be lent to the player?
If there are some cars, lamp posts, etc in the local area it is probably better that the player's intuition "just works." When it come to traversing large areas for whatever reason then the player and level designer need to be in agreements about what is easy, hard or otherwise to pull off. Intuition should allow players to survive in all cases but there should be easier and harder ways to do so.
There are basially two ways to keep players in sync with the designers intentions for a level. A waypoint system can be used or hints can be cooked into the level itself to accomplish the said goal, but which is better? Not all hints are equal but I feel subtle hints easily lend more to immersion and freedom. Hints like painting something a certain color or having it glow on command are not what I'm referring to. First some thoughts on using a way point system.
Take the latest PoP for instance. At almost no time do you have to figure out where you need to go because of the way point system. This is a good thing however... Many have said the game world feels "rigged" with certain paths you can follow...or you can die...except dying really isn't allowed either. The player always knows where to go but also can feel quite limited in what they can actually do on their own.
In contrast, the free-form world of Assassin's Creed allows players great freedom but the avatar never misses a grab/etc. All paths are the easy path for Altair as the avatar isn't ever worried about slipping.
Instead of ledges what if the avatar has to contend with slippery rocks, hot or near molten rocks, fleeting visibility, and surfaces that can snap or become dislodged when external weight is transfered to them?
Things get tricky or do they? Players can quickly surmise whether rocks are slippery, molten or not to solidly adjoined to a wall through the level's artwork. What this means is that players can use their intuition and with proper care the immersion of the player is heightened instead of cast aside to help players get from point A to point B. Too make things simpler lets create two groups of objects an avatar may use to traverse an area with group 1 being always safe and group 2 being variant with regards to being safe to traverse.
When presented with getting from point A to point B any combination safe and variant objects can be placed between points A and B. A simple mechanic would be to have more safe objects along one path between A and B but for that path to be longer, more accessible to enemies or what have you. Another path between A and B would have more variant objects along itself making it less accessible to enemies or the shortest path or what have you --- still, this path is innately more dangerous because the variant objects the avatar must interact with may case the player to be slowed to a crawl or at worse fall to death.
This system allows for free from traversal however this alone isn't anything new. This system is different in how the player is allowed to deal with the variant objects he or she may encounter. No game has introduced the concepts of weight, friction etc when it comes to environment traversal but now they can be.
A slippery rock is slippery so hands and feet can slide off of them. If a hand or foot slips the player needs to reset it slowing the player down. This is a gameplay mechanic that can be leveraged. A button press or a jab of a control stick towards the hand or foot could be used to reset the appendage. I prefer the latter as that's more visceral.
The same concept can be extended to heat or sticky goo on surfaces or what have you. A hand or foot slips or gets stuck and the player actively has to deal with it but in a fair manner. In some cases a slip etc coud be ignored altogether depending on footing etc. What's more is these "environment attributes" can be dilaed up and down and conveyed convincingly to the player with water running over rocks, heat glow or the increased sticky effluence of some creature.
Weight could be handled by sixaxis. If a variant object looks shacky the player could angle their sixaxis away from it so that the avatar's weight is centered on the other objects the avatar is in contact with. This would certainly allow for skill progression which is something avid gamers may enjoy. A route of escape/etc that is supposed to be difficult could become the easy route for skilled players where they could do a lot to reduce the chance for slips and falls using this mechanic.
None of this is going to be convincing without an extremely robust animation system.
Tie it all together well and we can get past jumping over crates, and spider man wall climbing. We may be able to get more immersive environments and gameplay by taking environment traversal to the next level.
I would love to see it anyway. I think all the pieces are in place now.