How would you do *actual* high-powered fantasy play with D&D?

[NichG]

Probably the easiest answer is to add a subsystem that all characters get access to, and that consist of 'big actions' that take longer to set up but have massively amplified consequences, compared to the 6 second baseline. Call them 'mythic abilities' or 'legendary actions' or whatever term isn't already overloaded with other stuff, create an in-setting justification for how they work much like pretty much any shonen anime will have some kind of power source that justifies the ridiculous scale of stuff, and then go from there. Have it be multiplicative rather than additive with regards to character abilities without the subsystem, to encourage narrower things that you can do even more extremely versus just 'all the numbers go up but its the same thing'.

So like, lets say we want to do this in a setting. We introduce something like 'vestiges of the energies of creation' alternately called 'Vestiges' or 'Creative Energy', and make some stuff about how some people may develop compatibility with this energy or collect it within their soul - especially those who are well traveled and experience more of the breadth of creation e.g. PCs and major antagonists. Tapping into the Vestiges requires setting the stage - the work of reshaping creation isn't something one does on a whim, but one must create something of the same scale that one wants to influence. A warrior might build a mercenary company or a citadel or something, and tie their Vestige Actions with that place; a druid might rearrange a bunch of ley lines to create a nexus of natural power sustained by the feng shui of the landscape; etc. Once you've done that, you can invest creative energy into that anchor point in rituals or repeated practices that take months, but once you've done it then you've created an 'exception' in the laws of creation that you can take advantage of to do-random-shonen-manga-thing-X.

One of the things about the shonen manga genre is that basically a lot of the scaling has to do with the side-effects of moves on the world. At the core, you can still have someone with a 70% chance to hit someone else and deal half their hp in damage, but now that blow also punches a hole in a mountain behind them or creates a tsunami or whatever. So rather than inflating the numbers to put everyone on a coherent scale (which is going to be awkward no matter what), you can basically say 'if you don't meet this criterion, things which do will basically oneshot you and you can't do anything to them; but if you do meet the criterion, fight as normal but with bigger SFX'. That's specifically for shonen though, which I wouldn't say is the same as high fantasy. For high fantasy, its more that the miraculous really can shape the world in big ways rather than 'everything is pretty much the same, but there are fireballs' - so specifically for high fantasy I'd put more focus on making permanent effects or changes. Here's a city where everyone is blessed with supernatural fortune (and here's how PCs could make that), here's a country where the empress shares her knowledge telepathically with every resident, etc, etc.