thegoldencalfre.wordpress.com/2014/12/04/res-knowledge-vacuum/
This blog here has lots of really good articles, arguing for RE that is rich in content and knowledge.
I don't know about countering extremism, but if we want kids to grow up into informed citizens who can read quality journalism about events in the news with ease, understanding and pleasure, and take part intelligently in debates about policy and geopolitical trends all over the world, it is essential to have
a good, sound knowledge about the history and current reality of today's religions.
I would like to see RE that covers all of the world's major religions (and at least alludes to some of the minor ones)their histories, different sects, how they interact with one another, religious reformations that have taken placeand a good sampling of the "cultural literacy" that comes with religion. Having at least a passing acquaintance with major stories and themes of the Bible, Classical myths and so on is invaluable for being able to read fine literature with ease, and without having to check the meaning of references that come up.
"I don't think there should be religious studies lessons. I think it should be called diversity. Kids could learn about mixed race families, dissability, same sex marriages, different religions. It might make it more accepting for them if their question can be answered in a school environment?"
We already have PSHE to do this. With limited curriculum time, we really, really do not need to have students spend yet another chunk of the week sitting in a circle talking about feelings (or, in the case of my PSHE classes, sitting around in embarrassed silence while the teacher tried desperately to get us to talk about our feelings). Ensuring that students respect each others' family arrangements is important, but this can and should be covered in things like assemblies and pastoral care in general. Curriculum time needs to be spent on learning.