Units are initially trained with their rank (unless they are a Champion or Sovereign, in which case they just "are" their rank). Levels are then gained through questing and combat.
I would rather that units leveled at the same rate as any other Champion, etc, however they just have less points to buy stats upon level-up.
Maybe some Champions can gain experience points faster (as a Special Skill) , maybe some champions will allow their units to gain experience points faster (as a Commander Skill) ... maybe some Champions will allow Units in their army (not including Champions) to be considered a higher rank for purposes of level up (more points upon level up)-(as a Commander skill).
The benefits that are directly tied to levels should be the same though ... like that special thing you get at level 3, or level 7, or level 12, etc.
I would also like to separate rank from level (I think this is already implemented).
like Draft (special case) -> regular (normal) -> specialized (veteran) -> exemplar (expert) -> elite
That way you could have a level 3 elite pikeman or a level 12 specialized swordsman. At 6 points per level, the Elite would have 18 stat points allocated, and 20 initial HP from his rank. At 4 points per level, the Specialized unit would have 48 stat points having been allocated, with 12 inital HP from his rank.
Draft: 8 HP, 2 lvl pts
Regular: 10 HP, 3 lvl pts
Specialized: 12 HP, 4 lvl pts
Exemplar: 15 HP, 5 lvl pts
Elite: 20 HP, 6 lvl pts
Champion: 30 HP, 8 lvl pts
Sovereign: 40 HP, 10 lvl pts
and simply have the same leveling system for all Unit-classes and Ranks ... merely a different number of stat points per level (lvl pts), as well as a different amount of initial HP.
To top it off, I would like for any "unit" (below Champion or Sovereign) to be able to "re-train" in any appropriate city with new weapons, armor, and items ... keeping their level and their stat bonuses gained through leveling, requiring the same time and materials as you would for training a new unit (maybe more training time depending on level).
In this way, your level 12 specialized swordsman can become a level 12 specialized Death Knight.
A more extreme alternative is to allow a unit to re-train their rank as well ... so a lv 12 specialized swordsman could become either a level 12 regular Death Knight or a level 12 Elite Death Knight. This would be ... explicitly interesting.
