Whatever rule you set, be transparent about it, be open to feedback about it, and enforce it fairly.
I think most powertripping mods end up being unlikable due to one of several reasons:
passes a new rule that people don’t like. Refuses to (or pretends to) listen to feedback when people get upset about it
enforces an existing rule more harshly against people they don’t like, often to the point where the mod would first decide that a user should be banned, and then try to find a justification for banning that user. (As opposed to the normal process, where a mod notices rule breaking, and then decides whether the user should be banned)
Complementing the previous point, gets into petty arguments. Mods are meant to be the role model of the community, and that inherently means being the “bigger man” basically all the time. You need to be able to take insults and know not to respond.