Pioneer spam is the biggest problem of this game at current version. I really don't think forcing AI to build less pioneers is the solution to this problem; it will only makes game easier since spamming pioneer no tomorrow is literally only viable tactic for early to mid game.
I thought about it, and thought about solutions like making pioneers expensive (like 150 gold per pioneers), unkeep or hard-coded limit on the number of pioneers and other things.
Then, when I was searching for position for creating city (good food with prod/essence), I realized the real problem lies on population being completely irrelevant resource in this game.
Currently, this game defies common sense. Only because of based on tile yield and buildings, a city with population of 10 can make things and train units much faster than a city with population of 200. This happens because MAYBE (yes, big maybe here) other than gold, pretty much all resource generation is decided by largely tile yields and buildings.
And because resources are mostly generated by yields and buildings, having 'more' of them is only viable way to increase resource generation, that means... more cities to build more buildings and take tile yields.
Thus having 4 cities with 10 population each is FAR better than having just 1 city with, say, 400 population with exception of gold production. But alas, since things take SO MUCH TIME to build stuffs, players normally play this game with 'none' tax, completely eliminating the population advantage.
These facts above heavily favors pioneer/city spam, even more so than Civ 5 (the game witch I absolutely hate due to city spam and stupid 1tpu). This really needs to be fixed because the game is not fun at all to anyone trying to compete with AIs. Here are suggestions.
1.) Get rid of 'tile yield' and bring back population as primary resource. With magical(?) yield number, the game defiles common sense. We are talking about civ-game, so for basic stuff, the game at least to follow real-life example (like.... more people working things create in faster phase... just how easy to understand and implemented?)
2.) Improvements from buildings should be all percentage-based from populations except food and rare resources. Primary resources growth, production, research and gold should be completely percentage-based, not hard-coded number. So actually a city with 400 population can outproduce 4 cities with 10 population.
3.) Keep the prestige as main initial factor for growth and keep it as global. This means, a number of cities should not affect the overall population growth AT ALL. In 4x games, 'positioning' alone is a huge incentive for spamming multiple cities already, there is no more need for more advantages for having multiple cities.
4.) Other than min-distance requirement, players should be able to build cities anywhere. Finding a good position is really annoying/stressful and sway the game difficulty too much just based on starting position. With a game using randomly-generated map, this is just bad idea. That said, there should be some penalties for cities built on harsh terrain, but other than minimum-distance requirement, please bring back the flexibility. With three suggestions above, there would be not much incentive for making multiple cities other than positioning factor, which will be also diluted by outposts.
5.) And please, separate unit production and building production. With recent add-ons, cities are already chocking up with making buildings all the time.