Pallen: One thing I've found that helps make rebels more successful when they agitate, is to open Victoria2/common/defines.lua in Notepad and modify the amount of militancy that they lose when they rise, and/or when they are defeated. When you open the file it's right near the bottom, and the way it works is essentially self-explanatory.
The way I have it set up is
MIL_TO_JOIN_REBEL = 6, -- Rebels over this will join a faction
MIL_TO_JOIN_RISING = 7, -- Rebels over this will join a general rising
MIL_TO_AUTORISE = 8, -- Rebels over this rise no matter what
REDUCTION_AFTER_RISEING = 1.0, -- After a pop spawns a rebellion, its militancy will be reduced this much
REDUCTION_AFTER_DEFEAT = 1.5, -- After a rebellion is being defeated in combat, its pop militancy will be divided by this number.
The top three modifiers I have decreased each by 1. This makes rebels more likely to form brigades in the first place. The bottom two numbers I have decreased so that when a rebel brigade appears, the militancy of the rising POP only decreases by 1; this makes it so that if the factors which led to their militancy are not addressed they will more quickly rise again. The bottom modifier, reduction after defeat, is a divisor. I think in vanilla it's set to 3, so that after defeat the militancy of the rising POP decreases by 66%. Setting it to 2 would cut their militancy in half. I have it set at 1.5, so it decreases by 25% only.
What I hope I have accomplished (I'm not sure how to test it, it's a broad change) is that nations which are unable to address the causes of their militancy will be dealing with rebellions more often, and if the causes continue it is likely that the risings will grow in strength over time, instead of backing down after one rising. On the other hand, if the causes for militancy are decreasing (war exhaustion decrease, or implementing reforms that they were agitating for) the rebellion will still cool off.
Hope that helps, and if anybody here thinks what I have done is problematic or misguided please let me know!