So there are a few threads requesting upgrades for Generals/Marshall. Some talk about using the books, others exp. Personally, I like the idea of experience, whats the point in naming your general, if you can't advance them. I have a two part suggestion.

1st) Generals/Marshall gain experience from battles, more if they win, less if they lose; more if they beat the boss camp versus easy camps and bonus if you conquer the island/ beet the adventure.

Things that could be upgrade would be:
1) Generals Combat Ability: Generals Attack, Specialty (attack weakest, splash damage, ext), and life points
2) Generals Benifits Ability: Buffs that affect his troops such as +% to life points, attack, ect
3) Generals General Ability: Travel time, Troop size, Recovery time, and Troop Recovery aka if you win and loose troops depending on generals lvl you get so much troops back.

2nd) To increase troops ability, that a Army Science building and Advance Army Science building be added to the game. The Army Science building would be available to players lvl 24+ and the Advance Army Science would be available to players lvl 60+.

Things that could be upgraded would be:
1) Weapons: This would increase units attack
2) Armor: This would increase units life points
3) Training: This would increase a units accuracy
4) Specialty: This would be a special bonus, added after a troop has been fully upgraded in the mentioned above categories

As this would only effect the final outcome of the troop, cost and time to produce the troop will not be affected.

using the Army Science building as an example

lvl 1 Upgrades for and would be available
lvl 2 Upgrades for and would be available
lvl 3 Upgrades for and would be available
lvl 4 Upgrades for would be available
lvl 5 Upgrades for would be available
lvl 6 Upgrades for would be available

However even if you upgrade the building to lvl 6, unless you are able to produce Cannoneers, you can't do research for them. Same thing goes for all the rest of the troops. The Advance Army Science building would have a similar set up.

Each upgrade excluding the specialty upgrade would be limited to either lvl3 or lvl4. The specialty would only be available only after all the other upgrades reached the maxed lvl and would be a one lvl upgrade.

This suggestion allows for two things. First, it gives the players a chance to not only upgrade their Generals/Marshall but it also allows for them to upgrade their soldiers. Second, by breaks up the upgrading process into two distinct categories Troops and Generals/Marshall, it makes it easier then to upgrade your combative units.