Just like Explorers and Geologists, Generals should have an upgrade tree using books. If necessary for balance, you could make existing troops a little weaker, and then allow the generals to upgrade to (and beyond) their current strengths. This could apply to black market generals, too. The upgrades could be things like:
- Faster travel and battle time
- Extra hit points for some troop type
- Additional damage for some troop type
- Increased accuracy for some troop type
- One troop type granted spalsh damage
- One troop type granted faster strikes
- Resistance to a particular type of enemy
- More damage for the general
- Faster recovery time for lost battles
- Bigger transportation load (without increased battle load)
- Reduced enemy accuracy
- Increased spoils from battles
- Chance to "sneak attack" (all troops granted first strike for first round only)
It would add some additional flair to existing battles and it would provide more opportunities for specialization and strategy.