Here's a suggestion.
For each battle that a General takes part in, they score one point.
The points can then be converted into General Experience points with the use of Books.
Manuscript converts 50 battle points into 1xp.
Tome converts 25 battle points into 1xp.
Codex converts 5 battle points into 1xp.
Each experience point adds 1% value to the troops which the General commands.
eg. a Recruit under a 10xp General would have 44 hit points, and do 16-33 damage (accuracy never changes), an Elite would have 132hp and do 22-44 damage. A maximum levelled 100xp General essentially doubles all hit points, and damage ratings of the troops.
Whilst this may not seem like a huge increase, it would take significant investment in Generals and play a tactical advantage as to which Generals you take into adventures. It shouldn't really overpower the game either given that you will need to spend a significant amount of resources to level a General that far. But for those that do, there is a decent reward.