The spontaneous, off the cuff style of DMing and the 'rulings not rules' philosophy was one of the things that really sold me on the OSR and related games in the broader RPG hobby. I love the freedom and lack of irritating “game mastery” mechanics, and needing to consult a table. Playing in a game of a more rules heavy system where every time you try to climb a rope or talk to an NPC in a basic conversation you have to roll some stupid fucking climbing or small-talk skill because it's there in the rulebook.
With this being said, Rogues are in a unique place in the OSR game world. My game features only 3 base classes; Fighters, Rogues, and Magicians. If Fighters are good at fighting, Magicians are good at doing magic, then Rogues should be good at the other part of this equation; Rogues are the answer to the final cog of the dungeon-crawling gameplay wheel.
Something that opened my eyes about tabletop games and how you can just make it work was the idea of nearly arbitrary roll under skill checks, there doesn't need to be a skill on a character sheet; the DM just gives a number and you try to roll under it to succeed. That's it. That's all there is to a skill check. This was just so interesting to me I started to think of how the class that's all about skill, the Rogue, could fit.
Specialization & Backgrounds
An issue I see with the skill check system is all about the specialization of characters and types of skills. Under a more universal skill check system like this, one Rogue character who grew up in the circus and worked as a juggler and acrobat for years and another Rogue character who picked pockets and broke into houses in a city as a youth would both have the exact same bonuses, based on their level, to acrobatic tumbling or picking locks and pockets.
Your first instinct might be to give background tables or elements in character creation. I wouldn't, for the same reason I let any Fighter be equally capable of getting their bonus to hit and damage and bonus attacks with all weapons if they be a pikeman from a disbanded brigade or a wandering samurai. It's the same reason any Wizard can cast any spell, even if they have some theme like Conjurer or Pyromancer. Specialization hurts characters more then it helps them.
In fact, making characters not have explicit specializations like this actually leads to more interesting roleplay opportunities. Say your Wolf Rogue, who lived his whole life as a bandit, started to play a slow song on a harp to keep a hibernating she-bear from waking up and mauling him and his looter friends to death. This becomes an immensely interesting roleplay opportunity- how did the Wolf know how to do that? It's a great moment, and much more appropriate to a ROLE PLAYING game then pulling out a DC table and stating that you get a +4 due to your background: Musician.
While letting the DM create the chance needed on the spot based on the difficulty of what the player is trying to accomplish, having a more ordered structure would help if these skill checks are part of the core gameplay. It is important that they are part of the core gameplay, or else you're essentially denying a Rogue player part of their characters unique toolset. It should be mentioned that any class can attempt skill checks, and you may even give out bonuses based on attributes, tools, or past character experience; it's just that Rogues get a bonus.
But what actually constitutes as a skill? What are you allowed to roll for? I would go for a metric that doesn't involve most combat skills or spellcasting, as well as avoiding knowledge or language based skills. Social interactions should be covered by Reaction Checks unless its involving a debate or trickery.
Eligible examples for skill checks; Acrobatics, tinkering with devices, navigating social situations, playing music or performing, sleight of hand and more. Conveniently, these are all things the Rogue archetype is known to do already.
There's also the nature of 'what is possible' which could start a lot of arguments when trying to figure what a character could try to accomplish using this system. Personally, I would tie the amount of 'realism' I am limiting the player character by to their level. Low level characters are more mundane where as high level ones can do obviously supernatural feats.
Just arbitrate a large negative malus from the roll under target number of the skill check. For actions that seem supernatural, this can be a negative number meaning nobody even has a chance of pulling it off. That is unless they're a high level Rogue with a high skill check bonus, letting them bring it into the realm of possibility.
Skill Check Rules
When you declare or are prompted to perform an action that requires coordination, skill, and luck above the regular ability of your character you will make a Skill Check. The DM will either openly state or secretly set a target number.
Roll d20 equal or under target number + bonuses.
Challenge Rating Guidelines
|Fair||10 <||Unlocking a standard door. Climbing over a low wall while chased.|
|Difficult||8 to 6||Winning a dart throwing contest. Disguise yourself without suspicion.|
|Extreme||5 to 3||Cut coinpurse, replace with bag of stones. Tightrope walking.|
|Monumental||2 to 0||Scale a sheer, slick surface. Split a fired arrow with a second one.|
|Supernatural||-2 to -6||Lullaby that makes a beast fall asleep. Move in absolute silence.|
|Mythic||-8 etc.||Serenade a storm to calmness. Escaping the afterlife. Running on water.|