I don't know your academic background but will share my subject selection strategy for CE-2018. I have an engineering background. I have opted the following subjects:

International Relations
US History
I. Law
Gender Studies
Sociology

First of all why IR?

IR is a quite comprehensive and vast subject. It not only nourishes the ability of critical analysis but also helps in covering major portions of other subjects.
It is focused on the United States(being a super power) and automatically cover US History syllabus post 19th century. Pakistan Affairs and Current Affairs are just extensions of IR (trust me). International Law is a sub-ordinate of IR in my opinion as you already cover foreign policy, democracy, territorial integrity etc in IR.

Now coming over to Gender Studies. It is quite a new appearance in CSS subjects arena. I went through past papers of Gender Studies and they are cent percent focused on women problems in Pakistan. I even solved those questions efficiently, though I hadn't touched the subject yet. Also the topics related to gender issues are in the hit list of Essay paper. So this subject might help you a lot in the coming essay paper in CE-18.

Sociology has a bad trend of scoring. But I opted this subject just because its syllabus overlaps little bit with IR and Gender Studies. Also it broadens your mind and provides you more material for 'social issues' questions that may appear in Pakistan Affairs and Gender Studies.

After 2 months of preparation, I absolutely have no regrets for opting IR. It has really helped me in improving my analysis and gave me good grip over history, current affairs, Pak affairs and I.Law.

Good luck!