Momentum/Doom Mechanic for OSR Play

(If you wanna skip directly to the actual mechanic, scroll down to where it says Momentum & Doom in bold)

As a GM, I strongly believe that being able to generate things on the spot not only supports player's choice and emergent play, but also allows the GM to partake in the thrill of discovery, that wonderful feeling players get to experience whenever they open a locked door, not knowing what lays on the other side. Nothing makes me happier than having new tools to help me make the world the players explore more dynamic and responsive. That's why I love the classic, sempiternal random tables and games that make use of them.

Another classic I'm fond of and that also contributes in making a game exciting for the GM as well as the players, is the use of a die (usually a d6) to determine a 50/50 chance of success for events and situations that are beyond the player's reach and not governed by any official rule. 1-3 something happens, 4-6 its opposite happens. And since I've been tinkering with my own sword & sorcery ruleset, I wanted to find a way to expand it a bit, by Crom! In a world of uncaring, possibly non existent gods, Fate plays a bigger role in the destiny of men, and I wanted something to reflect that.

To get some inspiration I checked the Momentum and Doom mechanic from the Modiphius' Conan rpg, and I found it suffering from the same issues that plague many other rulesets that try to do something similar, like pretty much all the Warhammer rpgs I've read.

Although I appreciate the intention, to create the idea of otherworldly forces tipping the scale one way or the other while giving both players and GM more tools to affect the world they play in, they all seem to implement it in a way that goes against the experience I'd like to create at the table.
In the case of Modiphius' Conan:

• Momentum is a pool of points the players can draw from to trigger special abilities or get benefits for a task being attempted
• Momentum is generated mainly by players succeeding at tests particularly well
• Doom is for the GM, and allows her to create "complications" for the players, like trigger NPC special abilities, make a bridge fall down, or generate obstacles that "take one or more actions to overcome" (sigh)
• Doom is generated by a bunch of things, including players' failing a test badly, players' use of Momentum to succeed at a test, but also features of the environment (like, putting a dangerous bridge in a dungeon could give the GM one or two Doom points).

These mechanics generate three major problems as far as I'm concerned:

• More bookkeeping - Players generate momentum (keep track) by making tests, then they have a bunch of special abilities (keep track) that can be triggered by momentum spends (keep track). Doom is generated in a bunch of ways (keep track), which the GM can use to trigger NPC special abilities (keep track), etc...
• Enforces characters' skills instead of players' skills - a lot of this Momentum and Doom stuff is tied to mechanical skills
• If a wall crumbles on a player's head I want it to be because of a die roll or because the wall was there to crumble from the start, and the player acted recklessly or didn't ask enough questions about the environment. Certainly not because I can make it crumble with a point a player gave me two rooms before by failing to pick a lock. The book even make an example of how the GM could use a Doom point to literally empty a players' quiver while a foe is about to charge her. Bye-bye tactical transparency.

So here is my take on Momentum and Doom, hopefully more in line with that OSR style of play I'm a fan of (I'm sure someone else out there already thought of this and in a much better form, so if you know of similar things please link them in the comments)

Momentum & Doom

• Whenever a player rolls a critical success, add 1 point to the Momentum pool. On a critical failure, add 1 point to the Doom pool (on top of the normal critical hit/miss stuff)
• Whenever the outcome of an event that is beyond the player's influence must be determined, the GM rolls a d100. 1-50 desirable, lucky outcome. 51-00 undesirable, unlucky outcome
• For each point of Momentum that exceeds Doom, the chances of a desirable outcome are increased by 10, and vice versa. Chances can never be less then 0 out of 100 obviously, or higher then 100 out of 100, doesn’t matter how much more Momentum or Doom is gathered after that.
• Momentum and Doom never refresh. They carry on from one session to the next

Example

A player misses with a bow in a crowded area. The GM rolls a d100: 1-50 the arrow hits a wall or a tree, 51-00 it hits a passerby. The roll comes out 45 but the Doom pool exceeds the Momentum pool by 1 so the chances of the arrow flying harmlessly by are reduced to 1-40. The arrow hits a passerby.

This can be also used to affect the chances of a business making a profit, an army winning a battle over another, a politician succeeding over the adversary, etc.

This way, critical success and failures, which already represent Fate's intervention in the actions of the players, also end up affecting elements of the world around them.