Everything would be as per the B/X rulebooks and my B/X S&S classes except:
1. Use a hit point system similar to Mutant Future.
Fighter and Barbarian use 1d8 / point of CON
Atlantean, Savage and Thaumaturge use 1d6 / point of CON
Thief and Sorcerer use 1d4 / point of CON
For example, a Fighter with a Constitution of 12 would roll 12d8 for hit points.
2. Use a THACO system inspired by Tunnels & Trolls.
2a. All classes have a base Melee and Missile THACO:
Fighter - Melee 15, Missile 15
Savage - Melee 16, Missile 15
Barbarian - Melee 15, Missile 17
Atlantean - Melee 16, Missile 16
Thief - Melee 17, Missile 17
Thaumaturge - Melee 18, Missile 18
Sorcerer - Melee 19, Missile 19
2b. Using the standard Bonuses and Penalties Due to Abilities from B/X:
Modify the Melee THACO using the bonuses and penalties from Strength, Dexterity and Wisdom.
Modify the Missile THACO using the bonuses and penalties from Intelligence and double the Dexterity bonuses and penalties (effectively counting Dex twice for Missile THACO).
For example, Barak the Barbarian begins with a base Melee THACO of 15. His attribute scores include STR of 17 (+2), DEX of 12 (nil), and WIS of 8 (-1). His adjusted Melee THACO is then 14 (15-2+0+1... remembering that a bonus reduces THACO while a penalty increases it).
His base Missile THACO is 17. This is modified by his DEX of 12 (nil) which is doubled (but still nil) and INT of 9 (nil) resulting in an adjusted Missile THACO remains 17.
Monsters and NPCs would use a base THACO of 20 minus HD, modified by the mercy or blood-thirstyness of the DM.
3. Using the Experience Tables for each class from B/X, when a new level is reached roll 2d8. If the result is above the new level attained, the character gains a bonus of 1 to the following ability (d6):
each ability has a maximum score of 18 - if a bonus would increase the ability higher than 18 the bonus is disregarded for this level.
For example, Barak gains enough experience points to reach 3rd level. The player rolls 2d8 and scores a 7 which is greater his new level of 3 so he gains a bonus to one ability. The player rolls a d6 and gets a 3 so Barak's Wisdom goes up by 1, from 8 to 9. The player will now recalculate Barak's Melee THACO.
It keeps the class structure of B/X while reducing the reliance on levels. Using the 2d8 to roll over new level would mean that there would be faster ability improvement at lower levels but much slower at higher levels. However, using the standard ability modifiers from B/X would mean that the ability bonuses would not always have an immediate impact on combat - except for improvements to Constitution which would impact hit points. Also, given that starting characters would begin with ability scores generated using 3d6 in order and the random nature of the ability score bonuses, it would be unlikely that any high level character would get a bunch of 18's.
I have no idea how this would play or balance out (especially for the classes with lower level limits) but it sounds kinda fun.