Perhaps I did not understand you correctly or explain correctly - I'm forgetting that this works differently in the newer games. In STW and MTW, any unit leader is a potential general. He simply needs to start winning battles and gaining command stars. The little icons atop of each banner disappeared in MTW, but it still works the same.
MTW extended this in fact and added v&vs (I think these are called "traits" in later games (?) and other stats which affect how well the general governs a province and how loyal they are to their faction leader.