FUBAR works simply, you roll three (or more) dice and allocate the results between categories of Success, Sacrifice and Story. If you have advantages, any positive success results are magnified. If you have disadvantages, any negative results are magnified. Positive success results manifest new advantages (or eliminate disadvantages), negative sacrifice results manifest new disadvantages (or eliminate advantages). Feedback loops develop as positives beget positives and negatives beget negatives. The results allocated to the story category, allow the player or the GM to define the way these feedback loops play out.
The whole thing began very freeform, no specific skills or traits, everything was defined on the fly. The first revision formalised the various skills applied to the different character elements, but it was still very loose in the way it produced positive and negative traits, and in the way those traits impacted back into the action results. The traits weren't defined, but in some ways this was a good thing. A loose and flexible system is adaptable to the way the players use it, rather than forcing the players to adapt to the system.
As a designer, when I run FUBAR it works great. I actually redesign the system on the fly to adpt to the story as it unfolds, but there are a lot of people who might attempt to play FUBAR who are simply intimidated by the looseness, and the need to complete a system with their own input. It's certainly not as loose as "Ghost/Echo" on which it was originally based, but there are still a lot of gaps that need to be filled before it's a complete and coherent game. I want FUBAR to be approachable, and to make the game more complete, but to maintain a degree of flexibility (because we still have no idea where a FUBAR tale might go).
To this end, I'm thinking of instituting a range of standard traits each capable of being manifested in play by a range of skills, and each capable of manipulating the end result of certain actions. Such traits instantly define the way the story might play out, but there will still be the option to produce new traits on the fly.
Example Trait 1: Injury (Negative Trait)
Gained by - failed Athletic actions, successful combat actions by opponents
Affects - Athletic actions, Combat, almost any physical activity.
Example Trait 2: Information (Positive Trait)
Gained by - research, interrogation, observation
Affects - additional research, social actions (blackmail)
These are just preliminary ideas at this stage, and I'm thinking of maybe a dozen standard positives and a dozen standard negatives, all interlinked across the various skills. There will always be the potential to make new traits n the fly to fill in gaps in the narrative, but with a framework of existing traits it should be easier to understand how they work in the wider system, and what need to be considered when creating new traits.
It needs more thought, but it's feeling like a good step to fill in one of the gaps that has been in the system.