i don't seethe point of adding many new units. As they'll have a very short life span before going out of date.
Maybe though, especially in the modern ear, have a few tech levels to each units, maybe similar to vetern and elite. So, there's a basic tech you research for a unit, ie, flight for planes. Then you get the two really basic plane types. Then, once you have some other tech, you have the option of upgrading the planes into something slightly better (longer range, or something). And this happens a few times before you develop the advanced plane units.
But I really don't see the point of cluttering up the game with 10 differnt types of plane, multiple tanks, etc.