Thoughts about population
Trin Tragula, I'd be interested to see your suggestions for modifiers focusing on growth rate (which I agree are usually too high in Paradox games) rather than tax modifier. I don't find +10% or +20% tax modifiers too high, personally, and part of my thinking in experimenting with a "rich" modifier was to make certain provinces more important and worth fighting over. The Seleucids and Ptolemies fought six wars over that rich cluster in Syria and Judea that I worried about in the previous paragraph, after all.
I have been working with the pop growth modifiers for EU3, where pop growth in fact refers to the growth of the urban centre of a province (so not quite the same thing as pop growth in EU:ROME). What I did was to set the base pop growth to 0 and then made other modifiers (such as buildings, tax and manpower base) increase population growth. The population number in a province itself I set to act as a malus on population growth (so the population will reach a point were growth stops by itself).
In effect I made urban growth dependent on a lot of things (such as the richness of a province, the size of a present CoT, the amount of provincial buildings (which I interpret as how developed the administrative centre is)). I also made things like looting, winter, etc decrease growth.
In doing all this I was mostly using modifiers smaller than 1 percent (with a few exceptions).
The idea came from when I was researching historical urban population numbers for cities on an Indian map I made for my mod and the MM game. Unlike EU3 the real world did not see a constant 3% growth but rather the historical population of the cities in question tended to increase and decrease a lot dependent on a wide range of reasons.
Now this is only partially applicable to Rome as population growth does in fact incorporate the entire province. There is also no such thing as a provincial base tax or manpower to act set the "natural" population size in EU:Rome. But I still think that making population growth dependent on many small things is preferable to having a constant growth like is the case in most paradox games. This is especially true as manpower and tax income is entirely dependent on population in EU:Rome.
In my opinion no province should have exactly the same population growth rate as any other, the whole point of them are that they're different. Climate would make excellent modifiers for population growth (one has to keep in mind that even when the population in question is that of the entire province the growth modifier is as much a modifier of people settling in as it's a question of actual increased birth rates/decreased death rates).
As I stated earlier I think population growth is a much better effect to model the fertility of land (which is what the climate modifiers seem to be depicting currently) than a tax modifier as the base tax is already dependent on population. Using population growth modifiers would also mean that certain areas would become more populous than others automatically as long as they aren't continually plundered/and or underdeveloped.
If one was to apply the same reasoning for population growth to Rome as I've done in EU3 (and this is a big IF, I suppose

) climate could act as the modifier that set the base growth of a province on which other modifiers are then applied such as if the province has been looted, how highly developed it is, etc.
Total population could still act as a negative modifier that grows as the population grows making growth stop at one point (dependent on other modifiers such as the climate the province has). As the player removes barbarian presence, appoints a good governor, builds irrigation systems and increases civilization values growth might then again start increasing (and with it the value of the province).
Keeping your major population centres supplied with grain will also be a lot more important using a dynamic population growth model like this one.
Additional modifiers that are already present that could be used to effect population growth in the game would be:
barbarian_power
civilization_value
no_governor
corruption
coastal (currently unused)
non_coastal (currently unused)
tropical (already does affect population growth)
mild_winter
normal_winter
severe_winter
blockaded
no_adjacent_controlled
city_population
core
same_culture_group
non_accepted_culture
different_religion
same_religion_group
occupied
under_siege
looted
revolt_risk
nationalism (probably better left out in favour of revoltrisk, using both can lead to unwanted results)
foreign_rebels
desecrated
regional_troops (this one should definately have an influence on growth imho)
construction_tech_level
civic_tech_level
religious_tech_level
As EU3 doesn't allow global population growth modifiers I've not experimented with those but ones that might fit Rome at a glance would be:
stability
positive_stability
negative_stability (probably just one or two of these)
civil_war
war
peace
war_exhaustion (probably better than war)
tyranny
any of the ruler_party modifiers though that would have to be carefully tested
These are all static modifiers. I'm sure if one wanted to go this route to create a more dynamical population growth model there's also plenty of event modifiers that may be used to good effect.
It's also my view that the omen that increases population growth in EU:Rome is very overpowered. Taking it repeatedly can increase your population by quite a lot quite quickly.
Sorry about the long post (especially as I didn't give any set numbers, if one is to adopt the design I propose a lot of testing would have to go into which numbers to choose).

Even if the idea of a dynamic population cap isn't something you want to do I think you should consider the use of climate modifiers instead of (rather than in addition to) the base population growth currently in the game as well as the use of population modifiers below 1%.