Low-Level OSR Weird Fantasy House Rules

After musing on the "common wisdom" that most OSR campaigns don't last long enough for PCs to gain enough experience points to reach levels higher than 10 or so (if even that), I was inspired to rethink my reluctance to run a campaign with a relatively low XP limit. And after reading some really good house rules over at the blogs Pits Perilous and The Bogeyman's Cave which focus on low-level play, I thought I might as well share my humble take on how I'd run an OSR game with a "weird fantasy" or "low fantasy" flavor. (I know I'm late for Halloween, but I'm actually very early for what Bogleech would call Halloween 2 AKA "Christmas.") This is heavily built from an LotFP chassis because that's what most of my previous weird fantasy house rules used on this blog, so credit where it's due, but you can hopefully use this with Basic D&D and most of its standard, popular retroclones/spinoffs/variants like Old-School Essentials or Labyrinth Lord.

General

• 3 classes only - Fighter, Magic-User, and Specialist/Thief/Rogue (or whichever term you prefer).
• All PCs begin as humans.
• No weapon/armor restrictions for any class.
• XP Limit of 300,000 - Limit of Level 9 for Fighters and Magic-Users, and Level 10 for Specialists.
• My Checkpoint XP rules are in effect.
• 7 skills - 1 in 6 chance of success by default, increased by certain Ability Score modifiers and the Specialist's skill points. A maximum score of 6 in a skill means you roll 2d6 and only fail on double 6s.
• Athletics (modified by Strength) - Climb Sheer Surfaces, Open Doors, etc.
• Search (modified by Wisdom) - Finding secret doors, Architectural features, traps, hidden items, etc. (possibly also Hear Noise/Perception check-style functions)
• Languages (modified by Intelligence) - When exposed to a language for the first time, roll a skill check to see if you know it. Characters start out automatically knowing the common language for their area. Optionally, they could also begin knowing one additional language, or two if they have an Intelligence score of 16+.
• Bushcraft - Ranger-type outdoorsy know-how.
• Sneak Attack - Damage multiplier
• Stealth - Hiding, Moving Silently, Picking Pockets/Sleight of Hand
• Tinker - Picking Locks, Disarming Traps, etc.
• Maximum possible HP given each level - no rolling for HP.
• Ascending AC: Unarmored=12, Light=14, Medium=16, Heavy=18, a Shield adds 1. Even the most heavily armored monsters probably shouldn't have an AC higher than 21.
• Fighters start with +2 Base Attack Bonus and improve by +1 per level. Others start with +1 BAB and do not improve beyond that. Roll equal to or higher than AC to hit.
• Critical hits (natural 20 on attack rolls) do maximum weapon damage. Fighters are an exception; see their section below.
• No alignment.
• If your preferred rulebook normally grants Thieves the ability to cast spells from scrolls at a certain level, use the following rules instead:
• Fighters and Specialists may cast spells from scrolls. The chance of successfully casting such a spell is equal to the character's Languages skill. Failure means at the very least that the spell was not successfully cast, but it is still removed from the scroll as if it were cast; a magical mishap may also occur at the referee's discretion.
• Specialists cast spells from scrolls as if they were a Magic-User of half their level (rounded down). Fighters always cast spells from scrolls as if they were a Level 1 Magic-User.
• Magic-Users cast spells from scrolls as usual, and do not need to roll a Languages check to do so.

Ability Scores

• Strength - Modifies melee attack bonus and Athletics skill. (Not melee damage.)
• Wisdom - Modifies non-magic saving throws and Search skill.
• Intelligence - Modifies magic saving throws and Languages skill.
• Dexterity - Modifies ranged attack bonus, AC, and individual initiative.
• Constitution - Modifies HP.
• Charisma - Modifies hiring of retainers and retainer loyalty.
• No Prime Requisites or XP modifiers.

Fighter

• Begin with an Athletics skill of 3 instead of 1 (still modified by Strength on top of that).
• Can carry 5 extra normal-sized items or 1 extra oversized item before becoming encumbered.
• Begin with +2 BAB, gain +1 every level thereafter.
• For Fighters, critical hits do double maximum weapon damage at Levels 1-2, 3x damage at Levels 3-4, 4x damage at Levels 5-6, 5x damage at Levels 7-8, and 6x damage at Level 9.
• Alternatively, if your preferred rulebook includes rules for multiple attacks, weapon mastery, feats, special combat maneuvers, etc. they should be exclusive to Fighters (with the exception of Sneak Attacks for Specialists), and they should be used in lieu of these critical hit rules.
• XP beyond Level 9 - For every level beyond 9 that the Fighter would normally receive (i.e. about every 120,000 XP after Level 9), the Fighter may increase one ability score by 1 point.

Specialist

• Hit Dice are d6.
• Begin with 8 skill points to spend as they wish, gain 2 skill points every level thereafter.
• If your preferred rulebook normally grants a "Read Languages" ability to Thieves at a certain level, ignore this in favor of the rules for the Languages skill.
• XP beyond Level 10 - For every level beyond 10 that the Specialist would normally receive (i.e. about every 120,000 XP after Level 10), the Specialist may increase one ability score by 1 point.
• If increasing an ability score would normally grant a skill point in a specific skill (i.e. Athletics, Languages, or Search), but the character already has 6 points in that skill, the character can put an additional skill point into a different skill of the player's choice instead.
• Optionally, give 1 ability score point every 100,000 XP or so instead.

Magic-User

• Begin with a Languages skill of 3 instead of 1 (still modified by Intelligence on top of that).
• Magic-Users can cast Cleric spells. All spells are now Magic-User spells.
• Magic-Users can make Protection Scrolls like Clerics.
• No "Read Magic" Spell - Read Magic is removed from the spell list. Scrolls, runes on wands and magic items, and other typical magical writing that normally requires the use of this spell can instead by read automatically by any Magic-User. This skill is considered a basic part of a Magic-User's training. Essentially, every Magic-User is under the permanent effect of a "Read Magic" spell at all times.
• No need to memorize multiple copies of the same spell.
• Example: A Level 2 Magic-User memorizes Shield and Sleep. They can cast each spell once or cast Shield twice or cast Sleep twice.
• Spells can be cast from scrolls/wands/staves that are too high in level to be memorized. Spells that are too high in level to memorize can still be scribed into a Magic-User's spellbook from a scroll or another spellbook, and thereafter be imbued into scrolls/wands/staffs at the usual cost - they simply cannot be memorized (and thus cannot be cast as one of the Magic-User's normal spells per day).
• XP beyond Level 9 - For every level beyond 9 that the Magic-User would normally receive (i.e. about every 150,000 XP after Level 9), the Magic-User may increase one ability score by 1 point.

Optional Goodies

• If you want the game to be harder, the first house rules to ditch would probably be Checkpoint XP and max HP each level. Next would be non-Magic-Users casting from scrolls (if your system allowed that in the first place), then increasing ability scores with XP.
• Regarding that last one, there's also the middle ground of giving PCs a random chance to increase an ability score every so many XP, rather than guaranteeing it; maybe they would have to roll their level or less on a d12, or they would have to roll more than the current ability in question on 3d6.
• Critical hits cut both ways, so cutting them might actually make the game easier, but I'm not 100% sure.
• Mind's Eyes should pair well with these rules. (I got the idea for this magic item by wondering how wizardly NPCs could become immune to a campaign's level cap, and if PCs could learn the secret to doing it too.)
• If you want the PCs to be bigger weirdos from the get-go, you can have them all roll for Paranormal PC Quirks at character creation.
• Here's something you might allow Magic-Users to do if you want to give the wizards in your setting a horrible tradition for settling their differences.
• If a player REALLY wants to play a Dwarf SO BAD, you could permit such characters at a cost.
• If you think Fighters need a little extra somethin' to spice them up, you could try giving them (and only them) a feature like this (or better yet, the "Mighty Deeds" of the Warrior class in Dungeon Crawl Classics). Or the Weapon Mastery rules of BECMI/the Rules Cyclopedia. Or multiple attacks. Just something else to mechanically differentiate them from other classes, you know?