The AI at Prince should be really hard.
No! "Really hard" should be reserved for Deity level. And if you say that Deity is not hard enough then the answer is to work on making Deity harder so that expert players get more of a challenge. But leave Prince as it is now for the more "casual" player. Prince is supposed to be an intermediate level. That's the whole point of the difficulty levels: to give different players different challenges.
I do think there are areas of the AI that should be improved. I just don't think the game on Prince should be "really hard".
On aspect of the game that definitely needs to be improved is the AI not upgrading units or falling so far behind in science that their units are woefully obsolete. I play on Prince but I can easily get 2 eras ahead of the AI, causing me to get mechanized infantry when the AI is still using pike shot and musketman units. It makes war completely trivial in the modern era. I don't care if the AI is a little sub par tactically but at least give it era appropriate units so my modern era units are not fighting medieval units!!!
A few possible solutions:
- Make latter techs much more expensive to prevent runaway science civs and/or introduce a mechanic where the player has to choose between science and other things. Maybe have the player have to assign pop to districts. This means you would have to choose which yield to focus on. If you assign pop to your campus, it means taking pop away from say your industrial zone, more science but less production in that city. This way, the player would have to make sacrifices. They could not just have more of everything like it is now.
- Give civs that are behind in tech a huge discount to help them catch up. Maybe techs that are already discovered by 2 or more other civs get a 50% discount or something?
- Allow civs to buy units from other civs even if they lack the tech requirement to build that unit themselves. This way a lesser civ could still buy a few advanced units.
- Let the AI cheat by auto upgrading units for free.
- Introduce more units that don't require strategic resources so that the AI and the player can still field a competitive army even without a resource. For example, you could have a "bronze age" swordsman unit that does not require iron. It would not be quite as good as the regular swordsman but it would at least allow civs without iron to have a basic swordsman unit. Alternatively, you could change how strategic resources work where no unit requires them to build but instead units just get a buff if you have the resource. So all civs can built the swordsman but civs with iron get say +20% combat strength to their swordsman.
I think this could help a lot.
The fundamental problem though is that civ6 is designed for the human player, not for the AI. The game mechanics are designed for the player to learn and hopefully enjoy but they are very complicated for the AI. If you truly want a tough AI, you only have 2 options: change the game mechanics to make it easier for the AI or use machine learning and AI to create your in-game AI. Doing machine learning is not cost effective for a company like Firaxis. So the only option left is to change the game mechanics to be more AI friendly.