Currently, I understand that troops will NOT be, currently are not; able to gain XP. A portion of xp earned by troops is given to the sovereign instead.
Ok, working this idea from multiple angles, and including the existing text in the game, here are my ideas.
If items will have a cost, be stockable, and discrete, then the need for unit design will be reduced. If they become an abstract assigned to a city, then unit design requirements are reduced.
1. Start with Training Kits dependant on resources and research. Notice the Scout Kit and the Bandit Kit available. These should be a basic unit with all required kit and skills. Expand this idea out to City Guard (researched under defense), Mounted troops (Requires access to Mounts and research), Armored Knights (requires basic heavy armor), Archers, etc.... Since troops are taken from a village or city production, you can probably also create Resource Production kits to generate the Twighlight Beekeepers (it requires a special kind of person to harvest honey from bees that live off of magical steroids.)
2. Select the minimum training level for a completed unit. Basically, researching advanced training is already put into the game and should affect the quality of the the troops developed with the cost of time. Figure about 10 turns per extra level of training. Give bonuses for better trained troops based on the kits they have trained up.
3. Assign the Extras that will be assigned to a particular model, such as Flaming Swords, Repeating Crossbows or Twilight Honey. Note that certain gear should only be equipable to certain kits, Steel armored scouts generally aren't stealthy. By assigning Gear to a particular level of training you can ensure your "angelic knights" will always be an elite trained force.
4. If higher training is researched, existing troops will automatically start training to the new level over a long period of time, say 50-100 turns per rank. New troops can be trained during creation to a higher rank for 10% of the time. I never want novice knights and will only "knight" them when they can be veterans or Elites. Additionally, when a troop "levels up", give me a list of templates I can assign them to.
In practice, I start my game and create a guard unit. I go to unit creation, select a guard kit, select any options I want a novice quard to have, probably none unless we can start factions with specific techs or gear. The guard unit will stay a green troop until I research Basic training. Let's say I give my basic guards Staves. I don't want to waste precious metal resources on untested troops. When Basic training is researched, the Guard units across the kingdom will start SLOWLY training up to Basic level. When they advance, I get a notice that Joe the Guard in my capital city is now Basic, I can check the qualified templates I have saved and/or create a new one for this game and assign it to Joe. Joe will then get a Pike and some armor. If Joe survives until elite, I'll promote him to Imperial Guard and give him some magic bling.
At the end of the game, I want more people in my Imperial Guard. I'll queue up some in my training list but, regardless of resources, I'll need time. Figure 10 turns extra for higher trained troops per level and you'll choose between fodder and highly trained forces that'll hurt to lose.
A further alternative is to pull a MoM and require facilities in cities for providing Magic Weapons, Higher Tech Troops, or Better Trained Troops.