This book, by ex-Bishop-of-Sheffield Steven Croft, is a punchy, well-put, highly accessible and relevantly practical reflection on the nature of church and how it can or should be rising to meet the challenges currently facing it in this country.*
At risk of overly simplifying what is an edifying and biblically deft read, his answer to the titular question - what the church should do next is exactly what it always has been meant to be doing, only with renewed unity, zeal, and diligent service. So - helping people from all over the place encounter personally the God of love by a variety of means, and building meaningful communities of these people around discipleship, which (because I know it's churchish jargon but I'm about to define it okay) essentially means encouraging and holding each other accountable to making Jesus the model of our lives and beings.
Simple, right? You'd think so. Then why do Christians keep churning out endless books about problems in this process or ways to overcome them, when according to generally orthodox belief all the answers are in the Bible anyway? I don't want to say I can't answer this question but I can't be bothered to type it out, nor risk such a rabbithole of potential theological debate bursting out in my comment sections, because as we all know this blog gets millions of passionate readers.
As a final comment - this book does also contain a really timely endnote on the current challenges of integrating 'fresh expressions of church' into the pre-existing and largely staler (sorry, everyone) economy of congregations, which is not in itself an overly useful or interesting bit of text but should go some way to pointing many churchfolk in useful and interesting directions.
* I won't discuss these in much particulars here, as I already put myself through far too much stress-inducing thought into such questions for my day-job.