Sure, sounds good. The book also introduces mythic play, which isn't level 20+ (you can start a Mythic character at level 1, funnily enough) which fits with the game design's lateral rather than vertical growth. This comes with mythic points, a Calling, a special track for mythic feats up until 10th level, and a 'mythic destiny' archetype starting at 12th level.
Mythic Points are the action currency for this mode - you use them to activate mythic abilities, spells, and so on - which replace Hero Points but work similarly. You get Mythic Points at the start of the session (a pool of 3 points, which is also your maximum to hold at any time), and recover them in play via overcoming mythic opponents or accomplishing mythic deeds, or by acting in concert with your Calling (which is essentially the narrative you've meshed yourself into via becoming a mythic character).
Mythic also makes the character harder to kill than normal - you treat dying mechanics as normal until you hit Dying 4 (or whatever modified Dying value would cause you to die, more on that in a moment) at which point you clear your Dying value and mark a stack of Doomed, and stabilize at 0 HP. This repeats, and you only actually die if you get whomped badly and repeatedly enough to have enough Doomed that your maximum Dying value is 0 - my copy of the PDF is saying Doomed 4, but I have to assume that would still be modified by Diehard and was an oversight that will be corrected.
So as I said, a mythic character has a Calling, which is your personal narrative and destiny. This comes with an attached edict (which when you follow it, gives you a mythic point) and anathema (which if you cross it, empties your mythic pool as a one-time thing), plus a series of actions that you can choose to spend a point on to roll with mythic proficiency (mythic proficiency is mod+level+10), and a once-a-day means of recovering a mythic point via critically succeeding on those associated actions. For example, the Artisan's Calling has the associated actions of Craft and Repair, an Edict to 'be the first to test an item you created/seek out or create formulas for ever more complex creations' and an Anathema against 'destroying the creations of other artisans/refusing earnest requests to repair something broken/using items created by others, except as a way to understand its construction so you can make your own.'
Mythic feats are basically just a mechanism for giving you things to do with your mythic points, and the lower level ones generally have an associated action - you can use mythic proficiency by spending a point, and often have an additional effect when you do. For example, at level 2 there's a feat called Binds That Tie focused around grappling that also gives you bonus damage on a successful check. At 10th, there's one called Eyes That See Eternity that gives you a minute of truesight and mythic proficiency on Perception checks.
Finally, there's Mythic Destiny, or to be more flowery the Paths to Immortality. Each of these are essentially a means of making your character into an immortal archetype. These are:
- Apocalypse Rider (daemonhood; you become one of the Four Horsemen, basically)
- Archfiend (any type of fiend other than daemons, player's choice; you craft your own hellish dominion)
- Ascended Celestial (self-explanatory)
- Beast Lord (despite the name, this is open to all types of non-familiar companions including artificial constructs)
- Broken Chain (immortal revolutionary; ideas are bulletproof)
- Eternal Legend (congratulations, you're a Heroic Spirit now)
- Godling (So You Want to Be a God)
- Prophesied Monarch (King Under the Mountain stuff)
- Wildspell (Wizard++)