I was thinking about how on occasion you just have a run of luck that seems to go right for doing one particular thing, and how you could reward that kind of fluky-ness. It’s not the sort of thing you would spend a feat on, but at the same time you want to have something that will effect the game and make clear the effect that seemingly random events can have.
For example, a character in a game at the moment has an unerring run of luck with their slingshot. I didn’t even want to take the sling as a weapon, but I needed a missile weapon and to keep the weight down, so I took it at first level, intending to drop it pretty much as soon as I got my hands on a crossbow.
Except that … my first use of it in difficult conditions was a critical hit. In fact, I had to get it out again when the party wizard ran out of spells and I gave them my crossbow. I kept hitting and hitting well with it. The dice loved that slingshot, and I wanted a reason to keep using it after a while.
Wouldn’t it be an interesting twist if characters could gain a new trait, say every four levels, that could reflect their happenstance development over the course of the last four levels?
Just an idea to chuck out there …
I've done similar things in the past, and it worked well. I think the main trap to avoid is making these reward traits or whatever you call them to powerful. As long as they stay relatively minor and balanced, they can add a lot of roleplaying flavor.
We had a very similar idea floating around in our group.
I think two things are important with this.
First these traits should absolutely NOT be given out at certain regular level intervals (it would just make it another leveling feature like feats). They should only be rewarded to characters that really have (real world) dice luck, either for that one certain, crucial, "all or nothing" roll that they somehow managed against impossible odds or, as you described, if they have an improbable run of luck for a relatively normal task. The important thing is the metagame element of dice luck.
The second point is that the bonus should be quite minor and more of a flavor element. Also make sure that for a single character there can't be more than certain number of these traits gained over the course of his 20 levels (1 or 2 at most). Since other characters might not gain any at all.
We also thought about giving negative traits or the possibility of losing one of these "luck-traits", if the character had exceptionally bad luck.
Interesting point.
If they were not handed out regularly based on level, how about on the completion of a chapter of an adventure? For example, in AP's you could gain a trait at the end of each module based on how your character performed in that module and what they had to do.
Note that I am not just saying that traits should be given out for a run of good luck - that's just one reason among many. There should be options for PCs who had rotten luck, or were not distinguished by any particular events in the game but still put in a lot of RP and hard work to make the game work.
If you give rewards for performance, then you're measuring the playing ability of real world players and not the characters (which isn't the idea).
I put the emphasis on luck, because normally luck stays in the background and doesn't play a role in the story. However sometimes mere dice rolls have a story changing impact and if this is something that can be attributed to a character it might be represented by a trait.
I say give them out for cool, and have them about the power level of a trait. The slingshot luck is a neat thing for the character, but other players might not be lucky in the same way. As long as you have a reason for giving them out, my experience is that players don't really care how many they get compared to eachother.
How many total you give out is entirely up to you. I know GMs who would give one out to each player every other session, and others who would only give 1-2 out over the course of the game. If you are comfortable adjusting your campaign to a slight increase in power, I see no real problem. Besides, while small bonuses can add up, they will not really change most of the game. Like the argument over how many point buy to use, most people just wont notice the added bennefit of an extra +1-2.
Ah, but you make the luck and the performance determine what traits they get awarded (and I stress, awarded rather than choose)...