So, the problem with Civ:BE (and also V and VI) is that they pretty much frontload all their bonuses. That means you get over a certain hump and the AI are just weaklings you're bullying.

The idea I had was a mod that gives the AI progressive bonuses as the game goes on. So like the first 20 turns or something, it's just base difficulty (of whatever you chose). Then each turn, the AI gets a 1% bonus * curve level (1, 2, 3...) * turn bonus. So at turn 100, they're getting a 100%, 200% or 300% bonus to resources that stacks with (base difficulty).

If you want to get fancy, you'd adjust it per resource type. So AI might get a much higher research bonus than energy bonus, so that they tech up more.