You can consider following subjects:
1. Environmental Science. It is scoring, interesting and overlapping subject. It can help you with GS & Ability too.
2. Gender Studies. It has shorter syllabus as compared to other subjects and it can help you with the essay as well.
3. Philosophy. Most of the answers will be to the point. If you prepare it well enough you can score highly. Moreover, it will help you in logical reasoning.
4. International Law. Again, it is very high scoring subject. If you choose this subject I would highly recommend using "Public International Law: Cases, Problems, and Texts (2010, Book by Dinah Shelton, John Cerone, and Stephen C. McCaffrey)."
5. Constitutional Law. It overlaps with political science paper 2. Since you have already taken POL SCI so this can also be a good choice. If you choose this one, do not buy substandard books rather use online sources such as jstor and lexisnexis.

I am opting Punjabi for CSS 2018. Can you please tell me what sources did you use?