The base change would be to use skin from "Enemy Race Manager", Depend on the time it can be in different stages of development,
one mod with 3 races and their tech tree, or 3 mods each with it tech tree.
There can be simple skin change for units, or complete rework, like You can only build units in specific buildings, and make parts in specific buildings too.
If there is a lot of time, specific new parts (ingredients) can be made for those units. - since player is only one human, for theran there can be basic IA factory to operate all vehicles, and in different buildings, You build vehicles to put them in, For Zerg, You make basic Larve, and depend in with building, You use different food or mutation ingredients, You get different Zerg, For Protos, Base would be blue flaming crystal, containing Soul energy, and You just build different shells for them to be put in.
