Custom Battles are a wonderful thing about Total War games — you can game out an army build before you actually have to build one in-game. However...
You can't always do this well. For example, if you wanted to see what Liu Chong's Chen Royal Guard would be like under the command of, say, Guo Jia (or a random/generic Strategist), you can't, as you can only give unique units to the characters that start with that faction, or historically belong to it. (Xu Huang with Cao Cao, Gan Ning with Sun Jian, etc.)
I suggest that you be able to define the faction to which each general belongs, or otherwise unlock all units for all generals (in the latter case, perhaps with a little icon denoting whether or not a unit is native to the original faction). In this way, the player can then truly experiment with different army builds given different characters.
While we're on the subject, it would be really nice to include:
- Generic Han, Bandit, and Yellow Turban generals, which will allow the player to test gameplay using generic generals' abilities (especially as unique characters often have their own unique abilities which are not representative of the class of general as a whole). This becomes all the more important for Yellow Turban factions, since there are very few unique characters and the basic generals' abilities are ones that you'd most often fight with.
- Play with captain retinues (especially considering Yuan Shao's focus on them, as well as that garrison armies are a significant part of the forces you'd fight with/against in the game, particularly in siege battles)
- Select forces to arrive as reinforcements (separate reinforcements or collectively, and from where they arrive)