This would be really easy and maybe a fun thing for you to do yourself. Having six different categories would make it kind of obnoxious to implement, but it's your idea, so run with it. I implemented a class system for Oldrim that had the player choose their major skills like in Oblivion and capped those skills at twice their character level and capped everything else at character level. There are balance concerns: not every skill is equally quick to level (smithing is huge pain in the ass, for example). It's also really annoying to expend a skill book's bonus point without actually getting anything for it because you're at the cap. But overall, I found it to be fun to play, and it did a nice job of creating a more focused concept. The problem with the static caps that you suggest is they place a much lower limit on maximum level than can be found in vanilla. With your exact numbers, the maximum attainable level before one would have to start making a skill legendary would be 41. Furthermore, since only one skill can get to 100, one would only be able to make that single skill legendary. This would not be fun.