Godowsky should have written one, or at least arranged one.
Actually, there are a few arrangements of Mussourgsky's Pictures at an Exhibition for piano and orchestra and they're pretty bad. I don't think I'd want to hear a piano concerto composed by him (and only him) much, either.
Schubert would have written a great piano concerto, I think. We can only guess.
Also, Alistair, do you have any piano concertos (or any concertante work for piano)? Based on the piano writing of yours that I have heard, plus your compositional prowess, I think a concertante work from you would be interesting.