5th Region
I have four of the five regions, missing only the Frosty Fjords region. All of my four regions are fully developed or close to it. My main city, of course, is also fully developed. I am at level 96, ...
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I managed to open the 5th Region within my first 18 months of playing. I had one region that was very population heavy (Cactus Canyon, mostly Omegas), at 3.7m, my others were at 2.8m, 2.0m and 1.5m.
It needs a lot of RZs and good boost buildings (Regional Hot Spots are excellent for this). I borrowed my best buildings from my Capital and bought a lot of extras during Black Friday - my last 1m population was just from swapping things around to get the best population boost, one small area at a time.
Once you make it you don't need to keep it over 10m, you can redesign and focus more on aesthetics.
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