I am intrigued by this idea, but instead of having 'levels' having 'tiers,' if I were to design such a system, I'd have character abilities be gated by tier. Mundane, Adventurer, Heroic, Paragon, Epic, and Mythic would be the tiers.
You could thus have games where the tier stays the same the whole way through, and the reward of adventuring is learning new abilities to widen your abilities, rather than increase your power. Or you could have 'zero to hero' games, or maybe something like 'hero to legend' plots. And there could be designs that require certain trials or achievements to go from one tier to the next.
And maybe there'd be designs like, "I'm a Paragon Warrior, but I'm able to pick up a few Adventurer-tier magical abilities. Our Paragon Mage also knows a few Adventurer-tier scoundrel abilities," etc.
That's how "classes" work in the homebrew game one of my groups plays. Classes are tiered. As you level, you can switch to a higher tier class. A Tier 1 Warrior can opt to be a Tier 2 Fighter at level up.
Higher Tier classes are stronger but have fewer action/fate points. Your tier counts against the Power level of the adventure for counting action/fate points. So you could roll with lower tier mundane character with tons of points to lucks your way of of jams.