With all the beautiful compositions that we can choose from in the vast vaults of classical music, I still sometimes wonder about what we might have had, if only a certain composer would have thought of it - or lived long enough to do it. And I am talking about something more specific than say a 10th symphony by Beethoven. Let me give you three examples of what I mean.
Schubert's Winterreise for baritone and symphony orchestra
The Winterreise (Winter travel) song cycle for male voice and piano still stands out as one of the best in its genre, and for me nothing topped it in that respect until Mahler came along. I have always thought that the bleakness of these songs would lend themselves very well for an orchestrated version. Schubert himself was no stranger to the re-use of themes and ideas from one genre into the other (for instance themes from impromptus for piano solo were recycled into a string quartet and an orchestral work), and had he lived longer, he might actually have tried this with a symphonic orchestra - the unfinished eighth symphony demonstrates how well he could handle orchestral textures by then. I was not surprised to find out that this idea of mine had actually been tried out, by contemporary composer Hans Zender, but I doubt whether the result is in line with my expectations for what Schubert might have done.
Brahms' clarinet concerto
There is no shortage of great clarinet concertos (Mozart, Nielsen, Finzi come to mind), but I think a Brahms concerto would have given them quite some fierce competition. Brahms composed two well-respected piano concertos, a double concerto for violin and cello, and one of the best violin concertos, so trying his hand at a clarinet concerto would not have been strange. Even more so, since some of his best chamber music revolved around that instrument, especially the two clarinet sonatas and the gorgeous clarinet quintet, one of my all-time favourite classical compositions. It is important that these were late works, and maybe time itself prevented Brahms from extending his new found love for the clarinet to a concerto. As a curiosity: Yuri Bashmet has transcribed the clarinet quintet into a concerto - but for his viola rather than clarinet (probably inspired by the fact that the Brahms clarinet sonatas are often performed on that instrument instead).
Mahler's Das Lied von der Toten Kinder
Perhaps the most likely of the three. Mahler's song cycle Kindertotenlieder (songs of dead children) for voice and orchestra by itself already has symphonic allure in its construction - moreover, the composer himself stated that these five songs were intended as one inseparate unit, and in performing them their continuity should not be interfered with. Of course, recycling songs into symphonies was something he has done frequently, and I can imagine that extending the orchestral lines of the five songs from this cycle could have created a symphonic master piece of the same standing as his famous Das Lied von der Erde.
Ahhh.... if only.....
The imaginary album cover I used as illustration is based on a painting by Friedrich (Monk by the sea). More on Friedrich in the wikipedia article linked to below.