Lets say that "John Smith the Imperial assassin" has just escaped from the sewers and is a VERY low level, and is clunking around with the rather bland Vanilla animations. After getting to lets say, level 25 in Acrobatics, John becomes an apprentice and now utilizes a new jumping animation. No more jumping like an ogre that believes it is a chimp. At the same time though, nothing too fancy. After quite a few more levels, he becomes an expert, and now obtains a flashy jump animation a'la "Amazing Jump".
Jane Doe the Breton fighter (a strange combo I know) has been a fan of one handed blades for some time, and loves the elegant and lightweight nature of rapiers and epees. She decides to wield one but much to her dismay she can for some reason, only swing it around as if it weighed two to three times what it actually does. She decides to train herself in the use of blades and and works to become more agile. After becoming an journeyman in blade and reaching 40-50 in agility, she can now use her lightweight deathstick much more appropriately by being more light on her feet in combat, and using "hit and run" tactics (Princess Stabbity-esque animations anyone?).
I have a few more hypotheticals but let me just right into it. Oblivion basically does this already with the weapon skills by giving you new power attack animations as you gain rank. Is it possible to accomplish the same thing with mods using a slightly different system? Jane Doe would have been swinging her rapiers around like normal longswords for a while but could a script "unlock" a set of animations after flagging her agility and blade skill after they are sufficiently leveled up, as well as checking the weight and reach of her equipped blade? We wouldn't want her to swing that much heavier longsword the same way she can with her rapier after all! A system that unlocks animations after skill, attribute, and equipment checks would eliminate the need for "special anims" and mods that only allow the player to use custom animations, by only allowing APPROPRIATE characters to use them.
Maybe this is just wishful thinking but a mod like this would be EXTREMELY immersive in my opinion. If anything here is unclear please let me know, because if something like this IS possible that would just rock my world.