Alright, I try to consider some stuff. Might be helluva long...
Anyway, proposed solutions :
1. Magic Theme, we modify the scientific weapon to be magical yet retains it's attribute to some extend. Guns can still remains, but generated through magic force (or blacksmith duh, Warcraft is Fantasy and those riflemen got some guns), but not robots.
2. Magic-Science Theme, either lower the tech power or get a boost for magic, which can be decided further on. There has to be a special logic applied to ensure the gap between two aren't high, using normal logic would favor one side.
Stuffs that is messy are like the long range nukes and controlled storms, time art spells, instant death stuffs, and other practically imbalanced stuffs.
Also, I was thinking if we better get a certain age based theme instead ; Renaissance, Dark Age, Steampunk... (Ok, go slap me now, gotta hide

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Personally, I can easily turn my factions into magic based or tech based or magic-tech based if I want, all of them. And I have no plan of using current factions anyway (aka start new)