Seems like an "obvious" suggestion, but couldn't find it on search, so here goes.
I think that each level for each skill should have stat level prerequisites to be able to get training from an NPC, which go up as skill levels do. For example, you should need more strength to learn the more advanced techniques of axe combat. Even just learning advanced dagger handling should require high agility, otherwise you can't even practice those skills. When you try to train without the prerequisite skill, the trainer would say something like, "Well, I can teach it, but you don't look like you have the dexterity needed for these advanced lessons. Come back a little later, when you're more agile."
Some skills would require a combination of stats: swords would be a medium-dexterity and -strength skill, for example, and blacksmith requires agility (to control the hammer blows) and intelligence (to determine where to strike with the hammer); you don't really need strength, you can compensate with more hammering.
I don't know any details of how the skills system is coded so I don't know how much extra complexity this would be for the coding team, but I think it would bring another level of complexity and (I hope) interest to the skills progression system.
