Of course, Mozart, like Bach, had a tendency to transform and improve on his models. On one occasion he even talked about writing an aria modeled on one by Bach's son J. C. Bach that was as different as possible from the original. He wrote "And indeed, mine does not resemble his in the very least."
Along with the Romantic tradition of extreme originality seems also to have come the "anxiety of influence" as in the theory of Harold Bloom. This holds that every artist (poet specifically) is terribly affected by their precursors to the extent that in order to escape this influence they will "misread" their precursors so as to achieve their original style. Mozart, if anything, may have had the opposite anxiety. Instead of worrying about being too influenced by others, he worried more about allowing himself to be too original. This probably had something to do with his family environment and the process of seeking a lucrative post for himself. But it also had something to do with Mozart's compositional personality. Music flowed from him like water in a river: he apparently effortlessly generated idea after idea, melody after melody. The main difference between his sonata forms and Joseph Haydn's is that where Haydn would, contra the theory books, tend to write a movement using just one theme, in a typical Mozart movement new themes keep appearing and he seems to be, in the words of Maynard Solomon, "seek[ing] to subordinate his melodic impulse to formal restraints."
But without any kind of conscious plan, Mozart did indeed develop his own style, though it was in no sense based on an invented "private language" as so many 20th century styles seem to be. Rather it was closely based on the genres and models of the music of his environment. One important genre as he developed his style was the serenade, a very popular celebratory and processional piece in Salzburg. Mozart's examples date from the 1770s when he was in his very early twenties. This is the Serenata notturna, K 239, dating from 1776:
It is charming and very pastoral in its evocation of the strumming of the lute and the rustic jocularity of the tympani. The serenade is one of a closely-related group of genres that included the divertimento and cassation. This was music that was intended to be heard once and then forgotten. Apart from the excellent examples by Mozart and a few others, nearly all of this music has disappeared. The traits he developed in these pieces, such as the ability to express nostalgia and longing for a pastoral idyll, went on to be used in more developed concert music such as the violin concerto. Actually, this gives us an interesting model of how concert music can evolve from more popular forms.
The connection between the pastoral style of the serenade and Mozart's violin concerto style is easily established as an aria by the shepherd-king Aminta from Act 1 of Mozart's 1775 opera Il re pastore is an early version of the first movement of his Violin Concerto in G major, K. 216 of the same year. The king sings:
Tranquil air and serene days
Fresh springs and green fields
These are the prayers to fortune
Of the shepherd and his flocks
...to almost the same music as the solo violin plays in the concerto. Here is the opening of the score to the concerto:
Incidentally, Mozart is the only composer I can think of that wrote concerto vehicles for himself on more than one instrument. He was not only a virtuoso pianist, but a very fine soloist on violin as well! Now let's have a listen. Here is the very fine team of Gidon Kremer with Nikolaus Harnoncourt conducting the Vienna Philharmonic: