The most two annoying aspects in single player games are these 2 situations:

1) The AI is too behind you by science, your units are 1-2 era above, so conquering them doesn’t makes any struggle 

2) The AI is already in medieval era and you haven’t researched wheel yet

So, in the first case, you have to pick next difficulty, but as you pick it, you get the second case, and have to choose to be dominated or dominate. But in both cases there is one period of time when everybody is equal and it is the most pleasant part of gameplay.

So, I believe there shall be option, that makes AI science equal to yours during all the time. So it is their choice what technology to open, but they have to be all time the same stage of development as player for the best immersion and emotions from game.

Also, this concerns the IP, as sometimes they have cool army you want to hire, and other time they stack in medieval era. I think the reason is that their development depends on average AI development, so if they all in past, then there are no reasons to hire those armies.

I also duplicate this on ideas section, do not hesitate to upvote it, if you liked it of course 

I agree that such large gaps between the empires make games less enjoyable.

The Scientific Correspondence Agreement is a good step towards more hamonized scientific progress. Do the benefits stack by the way? -10%Science cost reduction (exponential) for every empire that has already researched a technology would help. 

Without going that far, I made a thread about the idea, few months ago.