Anything at all similar to Civ V's Great Generals.
They make war ridiculous. And war is already ridiculous without them, being quite absurd due to the already-mentioned city bombard.
About them and about Great Admirals, there are some things that can make them better:
- Set a turn limit, a battle limit, and a 2 eras limit, to limit their duration . That way, you can't use the GG/GA you've earned in ancient era, used to warriors, arrows and trirremes, to improve the effectivness of tanks and destroyers .
- Allow their initial combating bonus to be small and then increase based on the types of units that the GG/GA is aiding, up to a limit . Eg: A GG can provide a +5% defensive bonus to every unit around him, +10% offensive bonus to any mounted unit around him .
- Also, another possibility is to make the Military National Wonder to demand either a GG or a GA born, to be built . That requirement could be expanded to other NWs, like spawning a Great Artist, in order to build the culture based NW
- At the end of their turn limit, allow them to be spent on any unit, to provide them an instant boost of exp, that directly correlates with the past experience of GG/GA . Eg: The GG mentioned above can provide instant +30 xp to one land-based unit and +50 xp to one mounted unit .
- Allow the speed of GG/GA to scale based on few technologies .
- Allow the GG/GA to teleport to any military unit, within friendly borders, upon spawn .
- Add a special quest for city-states, where they ask you to give away one of your GG/GA, if they are at war . It can be an UA