Have we ever discussed having Admirals (Naval Generals?) Could be very cool to do those and then include as Hydro suggests (and sets us up nicely for doing something for the Nomadic Start units that achieve a particular tech level to be able to purchase this kind of ability so we can have an accurate representation of Polynesian civ origins.)
This was my first thought when reading the idea of an ocean crossing promotion.
Could it be made that a GG can choose a promotion that lets that GG travel over water? And then have bunch of navy specific promotions depending on that.
Another branch could be added for the air force( affecting all aircraft stationed in the same city as the GG).
specific suggestions for that subtrees:
Admiral lets the GG cross water like a ship
Raiding I,II , III I gives a slight attack bonus on coastal tiles, II a slight increase in pillaging gold on water tiles ,III an amphibious assault bonus.
Fire Support Improves damage (or maybe colateral dmg) from bombarding enemy troops ( including opportunity fire). Could easily have several ranks.
Bunker Buster Improves city bombardment dmg ( those two could have a land version as well now that I think of it)
Battleship Focus Several stages, improving Battleships with combat boni.
The same could be made for other ship types.
Integrated air support Improves airstrikes. Not sure if it would be better to have it affect stikes that come from planes stationed within the control radious, or strikes that target enemies in the control radious. maybe both, as seperate promotion lines.
City raider Improved city defence and building bombardments ( this one should definitly have the GG near the planes, not the target.
Radar Operator needs radar tech, obviously . Increases interception chance for for fighters in control zone, maybe ground based AA as well.
I also propose a promotion line just focused on making the GG move faster. That way he could keep up with late era ships or tanks.
And another line improving the fortification bonus while reducing stack movement speed. Like the first rank giving -1 movement, and doubling the rate fortify boni are gained.
Rank II: loose another movement point, more resistance to colatteral dmg
Rank III: go even slower, but increase the max defence bonus from fortification.
For land GG: Siege Master in a few ranks, increasing surround and destroy boni
What is probably hard to do but would be nice: Synergies between GG ( or units in general). Like a promotion that is present both with a GG in the city the bombers start from and a GG at the target site giving a nice bonus for those bombings. Or a special "spotter" promotion that does nothing on itself ( ok, maybe +1 sight range) but becomes powerfull if the spotter GG near the target is combined with bombing, naval bombardment or the like promotions on other GGs whereever the bombardment comes from.
And something strange for units I just thought of: Scorched Earth: whenever that unit is retreating, it gets a 50% chance of pillaging the fought over tile.
