The title gives me an awesome idea- perhaps I might put it in Concepts.
What if someone made a map with a system sorta like
Fable where your actions would determine your enemies, and the storyline, etc. This could be done by introducing two scores- A Hate and a Love score. When you reach 0 Love and 0 hate, you are neutral, then you begin to go down either slope. Triggers would add a love or hate score to someone for certain actions. Quests would have multiple outcomes, for example a quest could begin in a certain way, but "forks" in the "storyline road" would add to hate or love. For example, lets say you have to slay Aster by command of Mini Moose 2707
. You then would be offered to join aster's horde. If you did, you would gain Hate points.
[eureka]
Perhaps after joining Aster's horde, you would gain a "hate aura" that adds a certain number of hate points per certain amount of time. Auras could be upgraded by completing certain quests or buying them. You could also have Love auras when doing certain things. Auras would be generated by joining certain organizations.
[/eureka]
Hate points would give you hate abilites and hate upgrades, perhaps different units for diffferent levels of "Hate". Evil units would be Zerg, good units Protoss, and neutral units could be Terran.
This could even become a campaign, with your Hate/Love etting the next scenario setting the next scenario.
This would even be a relatively easy idea to implement, just add a kills/deaths/build score to triggers and create aura players, which are much like the players used for looping music. The storyline forking is already an idea discussed by BeeR_KeG.
A downside of this is that the system might turn out to be buggy with people having both Hate and Love auras, however this could be fixed. Also, a level of antithesis might be annoying, Love this, Hate this, Love that, Hate that (sort of like
Metroid Prime 2: Echoes). The reason such antithesis would be necessary is side-specific shops to sell side-specific items, which must be an integral part of the game in order to get people to join certain sides.
Someone might have already got this idea, I didn't read all four pages.