If you had the chance to ask Mozart to compose something

[hornteacher]

How about finishing the K412 horn concerto by providing a middle movement?

[david johnson]

i would like a trumpet concerto...but i think it would be most interesting to say, " here's what's happened since you died.  use what you wish and do your thing, man. "

dj

[Maciek]

How about a large-scale piano variations works based on the opening theme to the Requiem?

Good idea!

Though a Cello Concerto wouldn't be bad either (he could tuck the variations somewhere inside )...

[quintett op.57]

A cello concerto.

Violin concertos being some of his finest works and Haydn's cello concertos being marvellous, I approve.

[mahlertitan]

a symphony in the style of Mahler (assuming he is a quick learner)

[Dancing Divertimentian]

I'd like to see him dive deeper into the piano sonata waters. With the doors that opened after Beethoven's arrival I bet he could write some F-A-N-T-A-S-T-I-C solo piano pieces.

Something chromatic, for starters...

[Steve]

A grand ballet in the spirit of Tchaikovski

[Symphonien]

Good point.  Another piano concerto . . . . . . . Only I'd request that Mozart first study the scores to Brahms PC #1 and Rach 3 . . . . . and then go from there . . . . . .

And thus have the key of D Minor in his subconscious...?

[JoshLilly]

I'd love to have this opportunity, but unfortunately it's not realistic.  What is realistic, however, is to find out from the Paris Bibliothèque du Conservatoire what the hell happened to the F Major Cello Concerto. They held it as recently as 1912, when they pulled the first 6 measures to insert into their catalogue. Those 6 measures still exist. The rest... what did they do to it?! I mean, it wasn't a fragment or anything, it was completed in March 1775. 1912 is fairly recent to be losing a Mozart work, though of course tons of music was lost forever in the two World Wars. You'd just have thought by then someone would have gotten around to making copies. Then again, Mozart's Cello Concerto is not the (probably eternally) lost piece of music to suffer from such lack of forsight.

There is the unfinished Triple Concerto that includes a cello soloist, a couple of people have tried to flesh it out, but consensus seems to be that there's not enough material to be at the point where it just needs to be touched up. Still, there are several completed versions out there, dating from the 1800s on up to more recent ones.

[Bunny]

If?  Definitely a Broadway musical. 

[hornteacher]

He could write the soundtrack to "Amadeus II: The Revenge of Salieri."

[bwv 1080]

Perhaps he could finish out Beethoven and Mahler's 10th symphonies

[BachQ]

Perhaps he could finish out Beethoven and Mahler's 10th symphonies

And the finale to Bruckner's Ninth . . . . . .