Thomas Sutcliffe: If only they'd listened to me...

Artists and writers justifiably hate it when critics review the work they wished they'd seen, rather than the one they actually did. It's understandable, really. Just imagine it. You spend months carefully crafting a table and then you open the paper to find that someone is moaning that the seat is too high for comfort and there are no arm rests. "It's not meant to be a chair!" you would shriek, "It's supposed to be a bloody table!" And one of the things that aggravates the artist's irritation, rubbing salt into the paper cut, is the sense that critics are betraying themselves at such moments – revealing their envy for the act of creation.

As I say, I can understand the reflexive wince, but it may be worth noting that the instinct imaginatively to perfect and make good the work you've actually been presented with isn't unique to working critics. Indeed, it's probably inseparable from any kind of intelligent encounter with works of art. It's just bound to happen from time to time, that you will look at a painting or read a book and think "oh ... if only" – that phrase summoning up a ghostly counterpart of the thing in front of you, one which has been subtly tweaked to meet your own bespoke requirements. It's happened to me three times recently – and each time it's been accompanied by an odd little pang of grief for the twin that never was.

The first instance occurred when I was looking at the National Gallery's exhibition of candidates for the next plinth sculpture in Trafalgar Square. I thought Anthony Gormley's maquette looked particularly promising – an oddly suggestive model of the empty plinth surrounded by safety nets cantilevered out on all four sides. What I liked about it was the ambiguity of these devices. You couldn't tell whether they were to prevent people from climbing up or something from falling down, and that seemed to me to charge the empty space above it in a very interesting way. And then – a little sheepishly – I discovered that Gormley planned to spoil this insinuating work by letting members of the public prance around on top. As we discovered this week Gormley won out, and I'm sure the result will add to the gaiety of the nation. But I still mourn a little for my version of the piece.

A slightly different thing happened when I visited Folkestone for its first Triennial and came across Mark Wallinger's piece Folk Stones. This consists of a large square of pebbles set into the clifftop lawns, the salient point being that there are 19,240 of them, each sequentially numbered in white paint – and that this number matches the number of British dead on the first day of the Battle of the Somme. This has local resonance, since many of those troops would have passed through Folkestone on their way to the Front, and Wallinger's field of bedded stones startlingly brings that number home to you – converting it from abstraction to reality. But looking at it I found myself niggled by an unhelpful thought. Wouldn't this make a terrific modern war memorial, with names cut into each stone rather than numbers written on in white marker pen? There's something about the interplay between similarity and distinctiveness that pebbles have that would be perfect for formal commemoration, which all too often loses a sense of individuality in a mere list. Since Wallinger didn't do it, though, it would be tricky now for anyone else to take up the idea.

That's at the heart of what you feel at such moments, I think. That the work as it is stands in the way of something you'd like to get to. I felt it particularly strongly watching Michael Frayn's play Afterlife at the National recently, about the Austrian director Max Reinhardt, who was forced to flee by the Nazis. What Frayn is interested in are the cross currents between Reinhardt's biography and Everyman, the medieval mystery play he staged every year at the Salzburg Festival. What I found myself interested in were the cross currents between Reinhardt and another master of mass spectacle, Adolf Hitler. And I felt a growing yearning for the play that wasn't there.

Rather than chain himself to the lurching rhythms of Everyman's rhyming couplets I wanted Frayn to get his characters arguing – about art, and emotion, and the theatre of politics, and whether epic spectacle has something intrinsically fascistic at its heart. Afterlife deserves all the credit for suggesting the idea but then, most unfairly, it also gets all the blame for ensuring that Frayn will never write it. It must be maddening – though there'd surely be something unnervingly passive about an audience that never arrived at the table and insisted on sitting on top of it.