I'm currently writing a sonata, and if I write the first movement normally with exposition, development and recap, but in 3/4 time and with that typical mm-BAP-BAP rhythm, would you still consider it as sonata form or would you call it a waltz? The second movement is just slow variations on one theme, and the third is a rondo.
A waltz is a type of dance, but need not be a specific musical form — although Chopin and Brahms, for example, used forms for their waltzes. A sonata is a musical form. So the two are not mutually exclusive. In fact, you might take a look at Chopin's waltz's in particular, because they might lend themselves to an expansion into a sonata form, either as a single sonata-allegro movement or as a multi-movement expansion.