Well, GCSE Paper 1 is out of the way, and there are a couple of weeks until Paper 2, with Paper 3 just a few days after that. Although nobody can predict with any certainty what will be on the remaining papers, it makes sense to focus on the topics and skills that haven't already been tested, and of course these will vary from one board to another and between the Foundation and Higher Tiers.
Starting with the Edexcel GCSE, the amazing people at JustMaths have put together sets of practice papers, complete with examiner reports and mark schemes, covering the stuff that wasn't in the first paper. You can find them at http://justmaths.co.uk/2017/05/26/best-guess-papers-yeah-right/
This post on Twitter by the equally amazing Corbettmaths lists the topics not yet tested by AQA; the videos referred to can be found at https://corbettmaths.com/ and the promised papers are now online at https://corbettmaths.com/2017/05/28/aqa-papers-2-3-predictions-2017/.
The Edexcel IGCSE is a little easier to predict as it's an older spec and there are only two papers. Key topics that have been predicted for Higher Paper 2 are:
Thanks go to the folks on the Facebook "Maths Tutors UK" group for the info in this post!