My idea is: stat strata. At the start of play, in Newgate, all your stats are in the least stratum, stratum 0. You climb up the strata, stat by stat, over time. A stat stratum goes from -1 to +3.
At the start of play you might have:
Dangerous+2 (stratum 0)
Watchful-1 (stratum 0)
Persuasive+1 (stratum 0)
Shadowy+3 (stratum 0)
Then say you do the shadowy thing and earn +1Shadowy. Now you have:
Dangerous+2 (stratum 0)
Watchful-1 (stratum 0)
Persuasive+1 (stratum 0)
Shadowy-1 (stratum 1)
When you go up to the next stratum in a stat:
a. You get access to new moves. Going up to stratum 1 in Shadowy, for instance, gives you "to sneak out of Newgate Prison, roll+Shadowy..."
b. It represents a concrete change in your circumstances, demeanor, and reputation. People can tell your strata by looking at you. You can tell theirs by looking at them.
c. Acting below your current stratum becomes easy, but acting in your current stratum is more rewarding. Like, I haven't been scared of urchins for a long time, but when the opportunity arises for me to steal their silk scraps, I pass it by. I can buy all the silk I need, now, by beating up people who aren't beneath me.
Does acting below your stratum count as stat+4? That might be perfect.
Acting in the stratum immediately above yours can probably just count as stat-2, and acting further above that as stat-3.
There's my idea! Now that I've written it out, it seems awfully obvious.
-Vincent