First, allow me to say, that a 'Great General' should be a very good addition to Civ. Now, for the specifics.
Great People usually have several, but roughly fixed ways to affect the game. Let's summon it up.
- Tech
* Give research points to a certain millitary tech. All the 'leaders' do this, so this should be a no brainer.
- Add to a city as superspecialist.
So, what should be the bonus? Let's consider the different options.
Add +x hammers
Add +% hammers to build up military units and improvements ONLY
Add addional XP to the units built in the city
Plus, possibly, adding some culture&gp points. Money and food would not really fit, though.
- Special specific action. Let's see first what the other leaders do.
Great Prophet: Build religious wonder
Great Artist: Cultural Bomb
Great Engineer: Rush building
Great Merchant: Commercial expedition
Great Scientist: Build Academy
So, we see the type of specific action is to build an specific building, or to add a great quantity of the appropiate type of commodity: culture (artist), coins (merchant), hammers (engineer).
So, two paths are open for militar leaders. Building or militar experience.
For building it should be simple enough. I would equate military academies with scientist academies. That is, turning the military academy national wonder in a g.p. induced building, with roughly the same effects, allowing its benefits for the city or cities that hosts the building (again, like academieS)
The second path would be, I think a bit of a 'military experience' bomb. That is, drop him in a stack of units and make them reach the next level of experience. It could be a great bonus, but I don't think it would be too unbalanced, even bringing all the units of a civ under it.
Upgrade everything can be feasible... but I think that would could have a more unbalanced effect in the game at certain points. Maybe, upgrade an specific type of unit for free.