My real suggestion has already been well-covered by Mouseferatu and delericho. I think any focus on tiers is basically a focus on raw power level, and thus time spent in the "mundane" and "epic" ends of it should be relatively brief compared to the heart of adventuring, which is "heroic" and "paragon". That gives 20 levels of "sweet spot" right in the middle of the game, clearly called out, but with enough levels on either end to play around there if you want. And it is just flat more straight-forward than kludging "zero level" or other early stuff before level 1 or completely and sharply changing the rules at "epic".
In such a framework, there is
some natural growth from grit to wahoo, but ideally that kind of tone is set by other things than power level--optional rules, restricted abilities, campaign restrictions, etc. Which brings me to my fun suggestion:
Steal the color coding from Burning Wheel, but use the prismatic colors to make it more D&D. In BW, "black" is mundane, "gray" is legendary, and "white" is supernatural. A handful of key rules change at each color, and when the colors interact, it is very difficult for someone on a lower rung to handle it. (If a dragon with "gray" claws gets after you, your "black" armor might as well be rags, for example.) It doesn't necessarily need to be that stark over seven prismatic colors, but the same principle applies. Roughly, the colors would represent a tone over the entire campaign:
- Red - extreme gutter rat grit. Chances to hit are often lousy, getting to zero hit points is dead as a doornail, and Fantasy Vietnam would be paradise in comparison. There is no raise dead.
- Orange - standard Fantasy Vietnam. If you live long enough to get some power, you might just make it.
- Yellow - last appearance of traditional save or die in this hierarchy. Things are generally rough, but you've got a few outs to mitigate (something like slight fudge, handful narrative "plot points", etc. depending upon how you want to play it) This is basically Orange with an occasionally benevolent DM or universe when things get the worst.
- Green - SoD still present, but muted and less frequent. Chances to hit/succeed have generally hit the solid 65% area. Foolish play or bad luck can still get you killed readily enough, but for the first time in this color scheme, you feel somewhat in control of your own destiny--instead of clinging to it while it tries to throw you off. This plays a lot like what 2E-style implied it did (not what 2E mechanics actually did, mind).
- Blue - You've got bonus hit point (say Con score) at start, generally can get any equipment you need with modest effort (which sometimes means you don't need it at all), and never completely run out of resources. Last point at which character death from random chance is at all likely, and resurrection isn't that hard.
- Indigo - You've got ways to outright nullify bad effects--"hero points' to make automatic saving throws (on the rare occasions when the playing field already tilted in your favor fails you), resurrection is a single spell away, and if you are playing Boromir, you break off the 6 or 7 arrows and grin before you kill all the orcs.
- Violet - completely wahoo. You die when it is time to die, and not a moment before. (This is explicit mechanically. Going to zero hit points leaves you unconcious, captured, etc.--always. There are no "death saves" or "dying" conditions.)
I might be somewhat off on the specifics, but that gives an idea of what I mean.
In some games, characters are able to change colors on select abilities as the game progresses. So you might have an otherwise "yellow" character who develops some "green" or even "blue" magic. If you want a "red" world but with heroes a "cut above", you make them "orange" or "yellow". This provides yet another way to differentiate characters without piling on plusses.