2. If heirs only die by event, and now that I think about it this is probably true, then I suggest the following:
Heir events should fire LESS often, and take into account the heir's age if possible (More likely to fire with a very young heir, less likely with an older one until they reach atleast 40), and there should be a 75% chance of triggering another heir event. If the new heir event can affect the heir age, it'd be good to see non 0 aged heirs. Heirs don't ALWAYS turn up at age 0, usually a little older, but ONLY if you've just had a previous heir take charge it seems. Anyway a new heir should probably be of a random age, younger than the last, although not required, as the new heir might be a cousin. I think if you solve the problem of a lack of heirs you also solve the problem of overpowered Royal Marriages, as you can only PU/Inherit a nation with a disputed succession and/or no heir. I chose 75% because it means you will PROBABLY get an heir, but with progressive firings of the event you are more likely to get a "no heir" event. (I mean not really but it only takes one... I think you get what I'm talking about here).
3. Suggestion regarding colonisation treaties, I thought it better to post it here. I figure it's a treaty, so there shouldn't be a relation hit for taking the province from the nation you have the treaty with (I assume this is right). Instead perhaps it should fire an event, where the nation recieving the colony can choose to buy it, or leave it for another time (Incase they can't afford it). You might also add the option to take it without paying for it, and THEN trigger the relations hit. I'm not really sure how the whole thing works or about the real historical treaty, but I thought I'd suggest it anyway.
7. I would advise increasing tradition decay, ONLY because it is the only thing really reducing tradition now. I'd rather the leader events DIDNT cost tradition. Perhaps when you write the leader events, the chance of an exceptional quality leader could be something like, Tradition * 0.5 or 0.25 or something. So higher tradition increases your chances of having a very good general.
11. OK here's what I suggest. Decentralisation represents more power going to local leaders. In the case of say, england, this would represent the earls and other nobles having more power over their domain. In the case of a republic, it probably represents regional administration having more power. For example in a merchant republic, it might represent more power being with the individual city states. High centralisation is obviously the opposite. Kings trying to exercise absolute control over their domain. Republics running everything through a central administration, like a parliament or a senate. In the case of a Kingdom, it makes sense for this to cause more revolt risk, as disgruntled nobles wish to throw off their king. In a merchant republic, the city states will want more freedom. In the case of a Republic, which would probably have better control over larger areas anyway, it represents a too distant city ruling the locals or what have you.
I do question at this point your rational between centralisation and stability? Presumably more centralisation should lead to MORE stability, not less.
Anyway, I suggest to control blob sizes, you could set a global revolt risk, or if you like, a local revolt risk modded by some provincial factor (Population size perhaps? More pop generally means a more important province). I would suggest that having a core shouldn't TOTALLY whipe out the revolt risk. 50-75% of the revolt risk, but not all of it. Enough so that a very large empire, even with all cores, will struggle if it doesn't have sufficient government. RR should also be high enough that you can't simply steamroll a large number of territories without constant revolts. You should have to wait to aquire a core and maybe a similar culture in that province. Some experimentation will be required, but you can probably cut down on that with a little bit of forethought, looking at the actual state of the game. I'd also suggest that certain government types, like Revolutionary Republics, Empires, and Revolutionary Empires be capable of holding a fairly large number of territories. I think you have to have atleast 50 provinces to form an Empire.
Non colony, brilliant

I'd hate to see colonialism failing because of the changes. Ofcourse, colonial revolts will probably be helped along by this change anyhow, due to max provinces etc. I wonder if you can increase RR based on distance from the capital? Can you increase RR in fractions or only whole numbers?
Also, have you got a To Do list? I found this very useful as a modder, to remind me of my planned changes.