Rebuilding the second half of the fighter by itself is a bold choice. If the concern is solely fixing a perceived mechanical issue, then I guess you're golden.
Not really. Your example text certainly works. You have always had this X inside you. It's just expressing now. ((Sorry, I've kinda forgotten the exact wording of what you wrote earlier, but I think that's the general gist))
So, no, it's not all that different. I do think that the notion of a new sort of sub-sub class, as it were, that grafts overtop of existing subclasses. Or some sort of Prestige Class for 10 levels would work quite well as well. At level 11, you are a Level X fighter/(subclass), 1st level Mythic Warrior. Your actions have triggered some sort of mythic fate - maybe you are touched by the gods, maybe you have tapped into some sort of warrior spirit, whatever. I'm not very good at that sort of thing, so, I'll leave that to others.
But, I do think that that idea of a 10 level fighter only prestige class would function. Plus, as an added bonus, you could easily build a bunch of different ones that mirror different "mythic warrior" archetypes.
The early level martials deserve interesting things to do too.
Low digit martials are not, in fact fine. They're boring and designed around limited imagination and a stern determination not to be fantastic at all in this, the self-proclaimed premier fantasy RPG.
If the rest of these fantasy characters get to be fantasy characters, the martials ought to be allowed the same courtesy. Somehow minotaurs and cat folk stop getting to do cool stuff because they're swinging a sword instead of hand-jiving and rubbing guano all over themselves? I say thee nay.
I honestly don't agree with this. The whole point of a mythic warrior is that it is about keeping the fighters up with the casters. In the single digit levels, the fighter types aren't lagging particularly at all. Maybe by 9th level, I suppose. 5th level spells have some pretty big guns - teleport, bigger summonings, so on and so forth, but, I'm willing to give a bit of wiggle room here.
This way those who don't want mythic warriors in their game have no problems - just end their campaigns when they normally end them - around 10th level. No harm no foul.
But, IMO, a "Mythic Warrior" archetype is meant for double digit, Tier 3+ play.