D&D historians know, of course, that the real 5th edition of D&D (counting Advanced D&D) already happened back in 1983, but there's still a lot of excitement out there over the new "5th edition" of D&D. Will they scrap the sham that was 4E? Will they try to recapture the 3E market from Pathfinder? Or will they bring their Old School Gamers back into the fold by going back to basics?
From my perspective, the answer needs to be the last one. OD&D never needed an overhaul, just tweaking. If you want tons of options, skills, or superpowers -- oops, I mean feats -- then buy all the supplements Wizards could come out with for the new edition.
The basic rules should be two thin softcover books: a Players book of about 70 pages with character creation rules, spell lists, the basic rules of combat and campaign play -- everything a player will need for up to level 12 characters; and a Dungeon Master's book of maybe 110 pages with magic items, monster stats, a fuller explanation of the game mechanics needed for campaign play, a brief sample of a dungeon, summary of a starting town, and an overview of the World of Greyhawk campaign setting.
The new core rules in the Players book should look a lot like the 1977 Holmes edition of D&D, only expanded out to level 12.
I would tweak ability scores so all scores of 14-17 would give you a +1 in something and an 18 would give you +2. Strength would add to melee damage, Intelligence would add to the number of spells you know, Wisdom would add to the number of spells you could cast, Dexterity would modify both missile attacks and Armor Class, Charisma would add to your max. number of hirelings, their loyalty/morale checks, and encounter reaction rolls. Constitution would add an hp bonus, but also give you a percent revival chance that would determine if you were dead or just unconscious every time your character reached 0 hp.
I would add half-elves, but keep all other races and classes the same as Holmes. Just the four basic classes -- Cleric, Fighter, Magic-User, and Thief. Fighters would pick up the ability to make a second attack with a bow, dagger, dart, fist, or hand axe each combat round starting at 4th level. Thieves' Hit Dice would bump up to d6. Clerics would not normally start getting spells until 2nd level, but could cast their bonus spells for high Wisdom as soon as 1st level. Magic-Users would stay as-is.
I would tweak upwards little things like the xp value of monsters defeated and the cost of platemail. I would add a little weapon damage variation, similar to the 1981 D&D rules -- daggers, darts, and fists would do d4 damage; bows, crossbows, all pole arms, and two-handed swords would do d8 damage; all other weapons would do d6.
Natural healing would occur at the rate of 1 hp per 6 hours of bed rest.
I would keep the d6 rolls for initiative from OD&D instead of by Dexterity, as per Holmes.
The list of Magic-User spells would be the same as Holmes for levels 1-3 and as per OD&D Book I for levels 4-6 (adding Ice Storm, Fear, and Monster Summoning II to level 4 and Legend Lore to level 6). The list of Cleric spells would be the same as Holmes for levels 1-2 (with Command added to level 1), as per Greyhawk for levels 3-6 (with Animate Dead added to level 3). Some spells that were broken would be tweaked, like giving a saving throw vs. Sleep and making Continual Light last only days instead of permanently, but most spells would stay as-is.
The Dungeon Masters book would include all the monsters and magic items from OD&D Book II and the Greyhawk supplement. The monster stats would look much as per Holmes, with some tweaks to damage and maybe an extra descriptive paragraph per monster. The sample dungeon would be a 12-room dungeon for level 1 characters (and a mash-up of the samples from OD&D Book III and the Holmes book). The town would be 9 pages of Citystate of the Invincible Overlord-level descriptiveness, detailing the Town of Hommlet, expanded from the original Village of Hommlet module. The overview of the World of Greyhawk would describe the planet of Oerth, the continent of Oerik, the region called the Flanaess, and then the Viscounty of Verbobonc, up to the year 585 CY.