Ok, my character thread is getting to be a little too elaborate and overthought. It really boils down to one simple question. I read this in the wiki:

"Once your major skills all reach 100, you reach your final level; no more increases are possible"

So, all I need to do is pick a single major skill I never use and I'll never cap out, right? For example, if I pick Heavy Armor, and never wear any heavy armor, then I'll keep leveling as long as I get skill points in everything else?

No, because you won't level if you don't advance major skills. You gain a level when you have increased any combination of major skills ten times. If you don't increase a major skill, you won't increase level.

Why do you want to keep leveling? Oblivion is not like other games. Since the world levels with you, you do not get relatively stronger; rather, once your primary killing skill is maxed, you will tend to get relatively weaker since many of your foes just keep piling on hitpoints as you level up. Every single thing in the game is available by level 30.