It's all a game of math and checks and balances when adding new units or if they created a race system tied into the RTS game.
For example, say you wanted to create 4 different races. Give them all the default units. now change them all up a little. For each stat you change for the better, you make a different stat worse. Example: Dwarven tanks move slower but dish out more damage, etc.
Replacing abilities on the other hand is where the bigger balance problems happen, but would help make each race feel unique once properly balanced.
Naturally you could adjust the differences in each race conservatively (so there's little difference in each race, but it's easy to balance) or liberally (so there's major differences in each race but it's harder to balance)
Personally I'd love if Dragon Commander had races tied in with the units, but with release so close that's pretty much impossible. Which kind kills a little of the replay factor cause no matter what choices you make in the single player campaign you'll have access to the same units pretty much. (And multiplayer will become stale that much quicker as there's going to be fewer possible strategies due to lack of unit variations)
For an example, imagine if you could only play as Terran in Starcraft. Easy to balance, cause everyone has access to the same units, but it lacks the unique feeling you get when you get to choose your favorite race to play as well as lacking in the replay factor as it gets boring repeating the same strategies over and over.
Last edited by Nokturnel; 01/08/13 07:42 AM.