The government (or agencies within the government) always make for good opponents.
Magic works. Magic can do things that technology can't. The United States faces threats to its interests, and its citizens, from supernatural agencies. Given these three parameters, there would be some government agency, somewhere, trying to make magic work for US interests.
Wizards, and the White Council, often seem to overlook that despite a mundane reaction to deny the impossible, there are enough people in the government that someone, somewhere, at some point, would have encountered a supernatural threat, survived it, accepted it...and had enough authority to do something about it. A 'spook squad' that is distrusted...but gets results, so it gets funding. And recruits. And may have already encountered Wardens, and considers them a threat.
Here's a nasty suggestion. The Black Council, although thoroughly ruthless, is acting as it does to gain control of the Council because they're aware of the government agency, and realize that sometime soon there's going to be a showdown between the Council and the Agency? Yeah, they're playing hardball; but they're up against opponents far worse. If they're not there to make the tough calls, then the Council goes under and it's full-out magickal war; mortals vs everyone else. Given the stakes, sacrifices have to be made...