Rebel generals!
I've always thought that it would be nice to (occasionally) see a rebel unit pop up with an exceptional general to lead them. History is brimming with such examples (why wouldn't one of the Mexican Zapatista rebel units... actually have "Zapata" as it's leader? The number of these "exceptional" rebel leaders should be quite small, but it would add something to the game. Can you imagine a rebel unit led by a general with -10 attrition, conquering provinces but not dwindling away to nothing? Even if one argues that many rebels wouldn't have set foot outside of their home province, wouldn't a great rebel leader attract new followers in every province they invade? (I mean this in the rhetorical sense... there's no need for that to actually happen, just that their unit size evens out between desertions and new recruits). I've just seen too many revolts in India, Russia, and Ireland peter out due to attrition, despite a complete lack of action taken by the home nation!
Also, and this is probably kinda silly, but hear me out: When a rebel unit captures a coastal province where ships are "in port", couldn't it be feasible for some of those ships to either be captured by the rebels, or for the crews to actually join the rebellion? Especially if the ships are "native" or "colonial"-quality. I got this idea while watching the 1966 Steve McQueen movie "The Sand Pebbles", where Chinese rebels repeatedly attempt to capture an American gunboat! "Rebel" ships probably wouldn't last very long, but it would at least cost the home country the loss of those vessels (and, maybe a loss of prestige?)