Was there ever any thought about using a skill based leveling system kind of like how the original Ultima Online worked. There were no levels just all the different skills and you gained points in those skills by actually using those skills. There was a cap as to the amount of total skill points you could have so you couldn’t just be the best at everything. This way would be more like reality where the more you do things the better you get and would also eliminate the need to classify any character as a specific class, if you got sick of doing one thing you could always just switch and do something else. Just think it would be a pretty cool way of doing things.
I think it already is inherently the case that you level certain skills over time, at least for the rune mage and warrior, because they require quite some training to perfect the runes/combos. And the more you perform a skill the more adapt you become in using it. @Logan just described it in this post.
For ranger and musketeer it’s more the general skill of choosing your ammunition and aiming and not so much skill specific.