In the past, James Maliszewski has suggested that the cleric class doesn’t have a lot of traction in fantasy literature, and that maybe magic users should absorb the cleric spell list (an idea which didn’t meet with universal approval).
It’s true, most sword-and-sorcery priests (most of whom are evil death priests) look more like wizards than anything else. Christian priests and monks appear in heroic literature, but they don’t have healing abilities: they may have the ability to banish demonic and fey influence, but so does a crucifix or a piece of cold iron, neither of which demand a loot share.
Meanwhile, a lot of OD&D people don’t like the thief class (which was introduced in a supplement anyway; and is a rare example of “power un-creep”.)
Hell, since we’ve already folded cleric into magic-user, let’s give every fighter thief powers! Backstab, climb walls, pickpocket, the works.
This does match better with pulp literature. The best thieves are also good fighters. The Gray Mouser is a great swordsman (as is his little buddy Gord the Rogue). Robin Hood is a good enough swordsman to beat everyone in Sherwood except Friar Tuck (who is a fighter, by the way, not a cleric), and from his archery ability Robin is obviously using the Fighter attack matrix.
Similarly, many fighters are good thieves. Conan is a thief. Fafhrd (and his little buddy Chert the barbarian) are thieves. Aragorn can Move Silently. Lord Juss from The Worm of Ouroboros has Climb Walls. Elric steals SOULS, guys. Maybe Sir Lancelot can’t backstab and pick locks… but maybe he can and it just hasn’t come up. What if Guinevere were wearing a chastity belt? Then he’d figure out how to pick locks quick enough. Plus, what’s the point of a knightly vow not to fight unfairly unless you have the capability to backstab?
Thief powers wouldn’t overpower fighters, either (although do OSR people even care about class combat balance?) Backstabbing is nice, but it requires a round of skulking around even in ideal circumstances, so it won’t replace normal fighter attacks – it would just be another situational tactical option. The trapfinding and climbing are a fun exploration-mode perk.
By distilling the four classes, we’ve basically created two classes: “supernatural guy” and “physical guy”. You could give them cool portmanteau names: “thighting man” and “clagic user”, I think, could get a lot of traction.
Cutting the bloated OD&D class list down to 2 is about as far as we can go, I’m afraid, before we can no longer define it as a class-based system anymore. Pairs are great, though. Two-legged stools can sometimes stand up in some conditions. USA has a 2-party system and it is doing GREAT. Gender hasn’t produced any noticeable problems or inequities.
You’re welcome for the ideas, Old School Revolution Guys!