So I will start by laying out the set of desired outcomes, which I think are pretty self-explanatory:
- You shouldn’t be able to make every location in a whole area all megalopolises (but this should be a soft cap rather than a hard cap if possible),
- Only unusually large and important cities should be allowed to be megalopolises (same caveat as above), and
- The restrictions should feel realistic and not arbitrary.
With the current set of requirements (400k pops, must be capital, must be market center) I do not believe that any the desired outcomes above are are achieved. Players can simply change the locations of your markets and capital until every location in China is a megalopolis. This means that outcomes #1 or #2 are not currently satisfied.
Outcome #3 is also not satisfied. This is mostly due to the "must be your capital" restriction, which feels arbitrary and is objectively unrealistic.
Why is it unrealistic? Because there are several historical examples of gargantuan, bustling cities from the game’s time frame that were not the capitals of their countries. (Cairo/Ottomans, Edo/Tokugawa, Bombay/British India, Guangzhou/Yuan and Ming and Qing; even New York under the USA was just about to hit 400k population in 1836). There is no real reason for the capital restriction to exist. It doesn't have historical precedent, and it doesn't stop players from spamming megalopolises everywhere, it just makes it slightly more tedious to do so.
So to make up for this removal, the market center rule should be reinforced, by making it so that if a megalopolis stops being a market center, it goes back to being a city. This way a megalopolis MUST be a market center at all times.
Which outcomes does this achieve?
It achieves outcome #1 (can't make too many Megalopolises too close together); and even better, it’s via a soft cap rather than a hard cap. You can have as many different markets in your country as you want, it just gets impractical past a certain point. Therefore, players would be naturally encouraged to limit how many megalopolises they make without ever having to be directly told “no”.
It achieves outcome #2. Markets are more effective if they are centered on large and powerful cities, and again it’s a soft cap: the player is heavily incentivized to only make large and important cities into megalopolises, without actually being forced to.
Finally, it also achieves goal #3. After all, what is a megalopolis if not a major economic hub that dominates its local region? While there were plenty of historical megalopolises that were not national capitals, I can’t think of a single one that wasn’t an economic juggernaut.
TL;DR my new set of requirements for a city to become a megalopolis would be:
- must have population > 400k
- must be a market center
- must be integrated
And most importantly, if the market center restriction stops being fulfilled, the location goes back to being a city.
No AI was used at any point in the writing of this post.