And what about the overpowered Ag trait? C3X gives us the option to do away with the despot exception and have them suffer the penalty regardless of fresh water. But with the penalty removed Ag will be on absolute steroids during the expansion phase, turning already overpowered trait into an absolutely broken one.
The Hall of Fame has an abundance of 20k entries with non-agricultural tribes. The fastest 100k game is with Babylon, with an entry 3 turns slower with the Celts. Russia has consistently shown up in lower level spaceship and diplomatic games. Also, there exist spaceship or diplomatic tables with Korea at the top, though I do believe Sumeria would have worked better (though not non-scientific agricultural tribes!). Conquest and domination tables have the Celts or the Iroquois often at the top, however, those tribes also have 3 attack, 2 movement UUs while no one else has such powerful units in the ancient era if I recall correctly.
For a research heavy game, Agricultural tribes still may seem to have stronger power, because of quicker expansion in the early game, and thus have a better economy and research potential earlier. However, scientific tribes likely lie close in ability if not superior in ability to agricultural non-scientific tribes, especially with rich enough AIs where technology gets sold for gold per turn at a significant profit.
If playing Always War or Always Battle defined as war always being the state at the end of a turn, and one has to pick between militaristic and agricultural, I'm not so sure that agricultural is overpowered also. The earlier and greater quantity of elites seems anything but a joke in my limited experience, if not superior, for spawning MGLs early.
If there could be constructed a way to make agricultural tribes have slower horseman, knights and cavalry than other tribes, I'm not so sure you would feel them overpowered. I guess another idea would be to remove horseman, knights, and cavalry entirely. Then agricultural tribes get only 1 movement (for horses or knights or cavalry) or 2 movement horse units (for cavalry), while non-agricultural tribes get horseman1, knight1, and cavalry1 just like classic civ III.
Edit: looking at the editor, for the agricultural tribes, their ability to make horse type units could get removed. Then they get some other unit instead of horses. Since many of the agricultural tribes are Native American, and they didn't have horses, this does seem like it could make for a more historically accurate game.
As another idea one might try buffing workers and/or settlers for non-agricultural tribes using a similar method by removing the ability of agricultural tribes to train these faster settlers and/or workers, and then giving workers and settlers a movement of 2 or quicker work speed. Agricultural tribes get regular class workers and settlers. Non-agricultural AIs with greater movement settlers could then expand horizontally more quickly, while agricultural tribes though still possibly having greater growth vertically would have less potential for growth horizontally. With different worker speeds non-agricultural AIs could make up the food gap also.
Also, instead of nerfing Despotism, what if you buffed Monarchy to produce 2 commerce for roaded tiles and water tiles, while Republic gets 3 commerce? With governments it seems even clearer the that the source of the problem is not that something is overpowered absolutely, but that something feels overpowered
relative to other potential things which fall into the same category.