I think Mozart's music would have become more like Beethoven's--had Mozart lived into the Romantic age, I believe he would have achieved many of the innovations that are not found until later in the century--we can only guess what he would have invented.
How can I say this? After all, most of Mozart's music is very light and exciting, very much similar to that of "Papa Haydn" (though of course in his own style). But looking at some of his later works, ie the C minor Fantasy, the Requiem, the Piano Concerto in D minor, and Symphony No. 40, for example ...
Especially in the Fantasie, we can see that he was exploring emotions to the same depth that later composers did, or even more so. We can only guess whether or not more of these would have followed--perhaps he may have become more like Schubert, perhaps--or not. Again, this is all theorizing ...