As Happy Valley drew to its exultant close, the talk began almost immediately about a second series. Will it come back? Should it? Or are there some things so neatly self-contained and perfectly realised that they should just end? It was a finite story, wasn't it? What more is there to say? As things stand the BBC says things "look promising" for a second series and writer Sally Wainwright sees the story continuing for Sergeant Catherine Cawood, played by Sarah Lancashire, although nothing has been confirmed. Of course they, and we, want more of a good thing. But I'm not sure we should get it.

Clearly, Wainwright should now get a free pass to write whatever she likes, every year, for all eternity. After the success of Scott & Bailey, Last Tango in Halifax and now Happy Valley, she's emerging as Britain's finest screenwriter in decades. But I'm not convinced her next project should be another round of Yorkshire-set violence, rape and anguish. As a story about a terrible crime and the officer investigating it, we're done here. Bring on Wainwright's next brilliant idea. I bet she has plenty.

There are some shows that are naturally built for the long haul and openly invite series after series of new characters and plots. But in other cases, a broadcaster's need to endlessly capitalise on a good drama feels like it can overrule its narrative need to just be "over".

The Returned … should it come back? Photograph: Jean-Claude Lother

I'm already a knot of tension about the renewal of two of my favourite drama series of the last year: ITV's Broadchurch and Channel 4's canny French acquisition, The Returned. With Broadchurch, I think they entirely resolved the plot and firmly closed the book with the final episode. No questions were left unanswered. And yet filming has just begun on series two with some exciting new people including Marianne Jean-Baptiste and Charlotte Rampling joining the cast. Obviously, I'll be watching because writing like that mustn't go unwatched. But I wish Chris Chibnall had used his enormous talent to carve a totally new story with new characters because I want to see what else he can do.

The Returned is a whole different story. It was all unanswered questions, so much so, that I can't believe they'll find a satisfactory way of answering any of them. I'm so worried that they'll try to nail down the totally compelling plot – the unexplained reanimation of a load of dead people from a French town – that they'll try to explain to me why it happened. What could they possibly say?

In this case, I absolutely believe that ignorance is bliss. I've never seen a series create such mass unease and fascination on social media purely because no one knows what the hell is going on. Mogwai's superbly unsettling soundtrack and the beautifully spare script had me from episode one. What will they do now? Start musing on the nature of what happens after death? Or even worse, definitively explain where it is everyone goes when they die? I don't think I could stand it. A total lack of specifics is what made it great.

Until recently, I felt this anxiety about the second series of Channel 4's brilliant conspiracy thriller Utopia. But now I've seen two episodes of the new series (due on screens in July) I am convinced writer Dennis Kelly is only going to add to the brilliance of the first. It takes a special writer to tread that path again without trampling the goodness out of the concept, but episode one in particular is bold, brilliant and a thrilling way to come back to that narrative.

One shining recent example of a writer quitting while he was ahead was Hugo Blick with his devastating noir thriller The Shadow Line. One perfect series. Fin. His epic new project, The Honourable Woman with Maggie Gyllenhaal, hits BBC2 in July and will, I think, make audiences glad he moved his attentions onto a new story.

Is there always merit in coming back to something, just because it was good? Or can the flogging of a successful concept actually ruin the memory of the original?