When West European civs are crippled, other civs should claim the land - Germany and Scandinavia at least are expected to do that (but sometimes Germany is disunited and Scandinavia is behind). Otherwise Russia, Carthage, Greece or Rome should be interested in small pieces of land, though with low priority. When it doesnt happen, they are busy doing something else. It should never happen anyway, because you must be very good to cripple ALL of them (England, France, Spain, Portugal, Netherlands).
Well it shoud be said that one problem with Civ in general is that what it takes to cripple a civilization is very little in some cases, and because your mod is designed to make smaller civilizations more competitive it makes them more vulnerable to this. Consider that Spain in most games I've played usually consists of 3-4 cities. If you are France and you build one at Barcelona, and prohibit a Spanish build near Valencia it holds Spain at only 3 major cities likely Santiago, Madrid, Seville, and perhaps Leon if they are smart. The problem is that with only 3 the Spanish really cannot afford the lose of even 1 city for any length of time. Since so much of this mod is about timing i.e. being able to extend into the Americas at just the right time in the game taking Madrid, or Seville can set the Spanish back 100 years or more without doing much else. The benefit to risk is extremely high given that the French don't have to do much of anything extraordinary. This sort of play works on other powers as well...take one of their most important cities when they have a very narrow base and you cause a bottleneck at just the right time.
Additionally, there are certain time periods you can exploit militarily specifically to do this. Usually by the later Spawns i.e. the Dutch Macemen will have spread far enough that you cannot make much headway since the defense gets considerably stronger but early on even Longbowman built en masse can have an impact.
Maybe the advantage I have found is that the damge to relations early wars do can be repaired but they can usually have a profound affect on the latter outcome of the game since there is less margin of error which is the exploit you are taking advantage of. For me I would rather have the dutch to deal with in America than the Spanish French or English since the Dutch tend not to build large consolidated empires instead opting for a spread out approach which I'll concede to them usually. But if I can take out a couple of the "big dogs" incidentally then that just increases the likelyhood I can build a large consolidated empire myself.
- In case you attack an AI that's having plague, it'll strike your units more badly than his.
Overproduction of units isn't a consequence of plague anyway. The AI works that way (without plague you'd see huge stacks) and I even tried to reduce this from the AI functions
Well that's the rub isn't is without aggressive AI the computers are too passive and innefectual but with it they pose only a short term threat, in the long term they cannot compete because their cities are underdeveloped and there is nothing you can do about that I guess...except have you considered designing your mods under the assumption that the AI is a good gauge for its strength whereas I tended to try to test assuming the opponents were human controlled. From that point of view the mod maybe less playable with AI players but more challenging when played against other humans.
With all this talk about the flaws of stability and the need to protect minor powers I feel it needs to be said that you seem to assume that there is country which must be minor. In fact with stability just about any country can be a major power if played right, and that is because stability tends to slow down the rate of expansion so that "first come" is not the most important thing. This is where I feel the real strength of stability lies in making players build and develop good cities not just "spam" cities. So in that regard we agree, IMO you just haven't considered that most of what is already done negates the spam advantage without the civ size penality in place. IMO if the city number penalty to stability was removed I would challenge anybody to try spamming as a tactic to win because I doubt it would work. Even if you could build enough settlers to carry it out before anybody else could get established you would likely have to fight off rivals from your colonies which would be a daunting task considering that 40 crap/dead weight cities would be too much to hold on to for a developed metropole of 4 cities.
That is why settler spam cannot work even without stability because 4 good cities are not enough to balance out too many bad cities and it takes enough time to develop the new ones to prohibit such a strategy.
So once again I would suggest stability be based less on the "size" of ones empire or its locality and more about how well that player is actually managing his countries. A player that can defend his territory, keep his people happy, and develop an economic/industrial base should always be able to expand IMO.