In a future TW game, the issue of "promotion" could be handled across the board by introducing some kind of "upgrade" mechanic whereby a unit gains new skills, equipment or abilities as it gains experience. Civ4 does that in a very abstract "gamey" way, but it may have a role in a more realistic historical wargame like TW.

For example, in a Napoleonic wargame, a French line regiment may be able to change formation faster or fire more accurately as it moves from green to veteran. (RTW had something like this, with later Roman units firing pila more promptly than earlier ones, as well as having access to tetsuodo formation). It might even be able to be "upgraded" to Young Guard, then Middle Guard and ultimately Old Guard. This kind is a sort of "levelling" mechanic that might be fun and intuitive to many players. It might help players get more attached to their individual units and, in moving away from the cannon fodder paradigm, assist CA in meeting their goal of fewer, more decisive battles.

It would be even easier to do in some other periods of history, which had more unit variety than Napoleonic warfare (M2TW has a fairly rich progression in troop quality and equipment).