If I were to revise Basic/Expert D&D or Old School Essentials for my own purposes (and that's the "edition" I favor), these are the changes I would make:
- The players roll their own dice for attacks, skills, and damage.
- Alignment only exists for supernatural beings. These beings embody their alignment. Some supernatural beings are non-aligned. Listed alignments for monsters and NPCs reflect tendencies to side with the forces of that alignment, but are not mandatory.
- Experience points are awarded for experiences in the adventure, not treasure and body count. (I'd probably use a variant of the system in DCC RPG.)
- All classes advance at the same rate.
- The value of precious metals is akin to that of Europe's actual medieval period. All price lists must change. (Also: coin sizes and weights are based on historical coins.)
- Combat sequence: Each side rolls 1d6. The side that rolls highest acts first. Individuals on a side act in the order of their choosing.
- Ascending Armor Class.
- Classes use "To Hit" bonuses. No attack matrices. (See Swords & Wizardry.)
- Unified saving throws with possible modifiers based on class or other factors. (See Swords & Wizardry.)
- There is no total dark vision except via magic. Some creatures can see better in low light conditions, but none can see in total darkness.
- Revise weapons and armor lists to be historically accurate in nature and nomenclature.
- Weapons made for smaller folk (dwarves, halflings) are reduced 1 die step. Weapons made for giant folk are raised by 1 or more die steps (ogres, giants).
- Any class can use any weapon, although clerics might be restricted according to their religion. Player characters may choose to specialize in one weapon appropriate to their class for which they get a +1 bonus to attack and damage.
- Revise spell advancement.
- Revise thief skills.
- Class hit dice as follows: d6 for magic-users, thieves; d8 for clerics; d10 for fighters.
- No level limits.
- No minimum attribute requirements for classes.
- Any player character kindred can be any class, but each kindred has both advantages and disadvantages. Some classes will be more common to some kindreds than others, e.g. thieves (or scouts) are the commonest class amongst halflings, whereas fighters are the commonest amongst dwarves, and elves have a higher proportion of magic-users than any other kindred. (Yes, I use "kindred" from Tunnels & Trolls to denote different species that can be played.)
O.K., this sounds almost like a completely different game (and a heresy to some), but tailoring the game to suit a group's preferences has been how many (if not most) people have played D&D throughout much of its existence. I spent most of my AD&D years resenting its limitations, but if I am to run D&D again, this will be the way I run it part D&D, part Swords & Wizardry, part DCC RPG, part Applied Phantasticality. That's the way I like it.
(Incidentally, I made many of these changes a few years ago when I started running Basic/Expert D&D again and it morphed into a modified Swords & Wizardry White Box, but I'm just trying to organize my thoughts for my own optimal D&D.)