I’d disagree with the other poster. You are already required to take some classes in Statistics for your PolSci major. You want to offer something unique to employers. Statistics is too similar to the major itself.
…it’s totally not. Political science majors usually require one course in statistics. It is usually not calculus-based, and usually teaches the very basics of statistics - in service to the goal that students understand articles and books written in the field (not necessarily that they are able to perform useful statistical analysis themselves). A statistics major will teach computer applications and the use of statistical software to do more intermediate and some advanced statistics. Even a statistics minor will require far more statistics courses than a political science major would, and would be a very useful complement to a poli sci major. They are two very distinct fields. In fact, that was the minor I was going to recommend myself.
But the answer is basically anything could be a good complementary minor to political science, depending on your interests and goals. If you are interested in political economy economics could be a good minor; if you want to do quantitative analysis statistics or math could be a good minor; if you are interested in the interplay between politics/governments and culture anthropology or sociology could be a good choice; if you want to write books or news articles about politics and government English or journalism could work; if you want to do international political work, a minor in international relations or a useful or critical modern foreign language is a good suggestion. You could choose literally anything.
So the question is, what do you want to do? And why does a simple major feel unsatisfying to you? Is it because you think double-majoring is the ‘thing’ to do or looks more competitive or something? And why a double minor and not just a single minor?