The level limit at 81 is supposedly intentional - Tiber Septum, as the lore goes, ruled 81 years. There's really no need to continue to level your character unless you want it to do something different - once you hit 100 in your main skills, you can still play the game just fine, and enemies won't get harder just because time goes by. There's no point in the game where you'll hit a wall with unbeatable enemies because you didn't level enough.
Some skills, like lockpicking, will probably never hit 100 for most people, as they require locked chests, which you can't just go find, it takes some luck. Leveling every skill to 100 is insane, no matter how you slice it. I'd hate to do the math, but its probably something disgusting play time wise in order to be able to achieve that.