Most strategy games have fixed units and possible behaviours.
However, think of a game like Magic The Gathering : each card is a set of rules. Regularly, new sets of card types are created.
I remember that the firsts editions of the game have been said to be prohibited in official tournaments because the cards were often too powerful. Later extensions of the game provided more subtle effects/rules in cards and they managed to balance the game apparently effectively, even if there is thousands of different cards possible.
I'm working on a strategy game that is a bit in the same position : every units are provided by extensions and the game is thought to be extended for some years, at least. The effects variety of the units are very large even with some basic design limitations set to be sure it's manageable.
Each player choose a set of units to play with (defining their global strategy) before playing (like chooseing a themed deck of Magic cards). As it's a strategy game (you can think of Magic as a strategy game too in some POV), it's essentially skirmish based so the game have to be fair, even if the players don't choose the same units before starting to play.
So, how do you proceed to balance this type of non-symmetric (strategy) game when you know it will always be extended?
For the moment, I'm trying to apply those rules but I'm not sure it's right because I don't have enough design experience to know :
- each unit would provide one unique effect;
- each unit should have an opposite unit that have an opposite effect that would cancel each others;
- some limitations based on the gameplay;
- try to get a lot of beta tests before each extension release?
Looks like I'm in the most complex case?