But these footprints basically = manually pixel painting a sprite for each creature. There's hundred of creatures. Whenever a new one gets added, new footprint would have to be manually painted.
I doubt it's particularly hard to write a program that creates these images automatically from b&w images of animal footprints found online.
You still have to add it to every single new creature, but it's a matter of searching for or making a normal track image. It doesn't need to be pixel-per-pixel, just a regular, rough b&w image.
As with density values of materials, this could be something that the community can help with.
Anyway, while this does make it easier, your point obviously still stands. It doesn't add anything that couldn't be covered by a text saying "you see human footprints" or "you see tracks of a small animal". In fact, you could convey skill-related vagueness better with text.
But as it seems to be right now, it wouldn't use the adventurers skill, but the player's skill (Same for sneaking).
So it's mostly a gameplay design question.
Personally I really like the idea, because it would be a great feeling actually learn the real track appearance of creatures, just from playing Dwarf Fortress.
On the other hand I'm not sure if it really fits in with the rest of the game.
EDIT:
As Askot said, RPGs are either character skill based or player skill based. And inconsistencies between them tend to sully the gameplay experience when the game has deep mechanics. It might not be a problem with light casual games like Skyrim, but I think more complex games benefit from coherent and consistent gameplay design in that regard.
However, I think there can be a solution to incorporate both things in games generally:
Everything is basically player skill based, but character skills define how the character executes the player intention. In other words, the player is the brain, the character skills determine the "muscle memory" or sensory perception (and the attributes determine the physical properties of the character body).So, for example, the player decides where to strike, and depending on the character skill your adventurer hits or misses. And depending how strong he is, he will make more damage.
That's actually already in the game like that.For sneaking this would mean, the player decides where to sneak, when to stop, etc, but sneaking/perception skill influence how much noise the character makes, how well and acute he can stop and, say, remain motionless, how accurate he can see where an enemy looks, who is alerted, etc.
For tracking, whatever perception/tracking ability he has, decides finding tracks in the first place, and possibly helping player memory with additional information like how old the track is, in which direction it goes, etc.
If indeed implemented like that, I don't think the concerns of Askot, that the skills become less meaningful are justified.
After all, it works for combat very well already.