Putting this here as a placeholder for an idea I had today. Should enthusiasm continue I'll continue working through it and adding to it.

To cut a long story short I'm a big fun of high-powered, "epic" D&D (having once been a regular on WOTC's Epic forums). I've run a number of 3E D&D games that involved levels 20+, and/or regular games that were high-level but just particularly over-the-top. In most cases, it turned out to be a huge pain as the game system started burning out under the weight of unbalanced character powers, crazy numbers of dice rolls, and excessive book keeping because building a character involved choosing 50 different spells, ten feats, combining 3 different PrCs and spending 1,000,000 GP on magic items.

So, what I'm thinking about is making an RPG that emulates super-high-level D&D but with a minimum of bookkeeping and such. Concept-wise, I'm thinking of mixing epic level D&D character builds and concepts with 'optimized' builds as well - probably this would involved race-as-class with some of the crazier 3.x builds (Hulking hurler/war hulk) being essentially refactored as "core" classes.
A number of "prestige classes" and overpowered racial options would probably end up as 'feats' or something like 5E's "Builds". For instance, there'd certainly be a Wizard class and then 'Lich' would be a wizard build option (which would give an Undead feat granting the undead type benefits).
Likewise Fighter would probably include a 'Tempest' build that includes a Two Weapon Fighting feat (equivalent to the normal TWF entire feat tree) plus extra benefits, alongside Supercharger and Arcane Archer builds. Other OP races like Minotaur, Half-Fiend, Half-Celestial, Mind Flayer, Were-Creature or Thri-kreen would probably just be feats as well, avoiding the assorted ECL issues in 3.x.

Numbers would be generally scaled-down from 3.x - for example level would range from 1st to 10th with an understanding that 1st is equivalent to 21st character level and 10th to about 30th. Multiclassing therefore would be handled with feats as well - probably more similar to 4E multiclassing than anything, unless I build one class that's more flexible specifically to duplicate multiclass options.

Monsters would include a bunch of the normal Epic Level Handbook monsters such as LeShay, Elder Titans, Demilich, Neh-thalggu, Mu Spore, Colossus, etc. Probably would throw in FR's Sharn, Phaerimm and Elder Orbs, too.

I'm actually open to core mechanics - leaning toward D20 since that's what D&D uses, but I could actually base it more on Dungeon World or even FUDGE, potentially.

More later as I think of stuff. I could potentially use more ideas on what sort of concepts should be 'core' classes/races/builds to start with.

One major issue with Epic in retrospect is where everyone has huge attack routines - one guy has a turn and its 5 attacks, then someone else has 3, then the next guy has 8, then the wizard casts a couple of spells, etc.
First I pondered whether it would be better to break down combats so that people got multiple 'passes' each round, and then I realized that..."when everyone is special, no one will be". I might as well just keep everyone at one attack, and assume that epic rounds are faster.

Potentially characters just get more attacks vs. lower-level targets (though that's traditional anyway).
(letting fighters sweep through (Level) extra less-than-1-HD opponents a round was an older-edition rule of course, but if an epic round is 1/4 a standard round, the epic fighter might get to roll roughly a d6 to see how many low level stooges they obliterate, instead of a d20).

Another alternative be a Speed attribute which gives an initiative adjustment based on (advantage over opponent), with a high Initiative roll giving extra attacks.

I'm slightly torn on this approach since a) it doesn't quite work for spellcasters, since they don't usually scale up to 4+ attacks/round so quickly and b) it might also be more fun to instead move to an abstract combat model. Maybe a dice pool where a character splits their attack dice across multiple combat moves.