So, really, you could just use FreeCiv code similar to Civ2 or Civ3 and change a few things. The setting and interludes were completely different but the gameplay isn’t too different.
It also supported modifying the terrain in various ways (with the worker-equivalent unit, big enough bombs, or there was an option to vote for global warming I think). You could use it to sink cities or change weather patterns (make your land greener by raising a ridge to block the rain). I think the weather implementation was really basic though, just a uniform prevailing wind you could block, nowadays we could do better...
Then there's stuff like the government system which wouldn't be super hard, but would be a whole new screen to add.
