1) What do you believe are the key qualities that one should have for it to succeed in creating a worthwhile overarching antagonist?
The actions of the antagonist need to be far-reaching and clear. An all-powerful antagonist sitting in some citadel of evil in a corner of the map won't work. He/she or they're minions should be actively being out on the map and making a clear threat of themselves. Potential terrorist-style attacks on city's, corruption of certain terrain areas and other stuff so people actually feel that some overarching power is threatening them. Also great ET opportunities in there.
2) What do you believe are the qualities that one should avoid in an antagonist?
As slightly mentioned before, if you have an antagonist you need them to be active and a clear threat, always looming over the people. If he
/she/that becomes some kinda bad thing I guess just sitting somewhere far away, there might as well not be an antagonist.
3) What do you believe an antagonists job is?
To create suspense and a sense of meaning in the world. Also especially in these times, we need someone people might be able to fight together, as in the races, so Oren might stop beating the **** out of everyone because they're bored.
4) What do you believe would give a more enjoyable experience, an overarching antagonist entity or a splintered group of antagonist entities, or something else?
There should be a single overarching antagonist. This person should have some powerful minions of chaos, who can go out and actively create chaos in the world.
5) Any other information you would like to give on the topic?
Hit me up if you need building manpower.