The ending provided in published editions of Mozart's unfinished D-minor Fantasia, K. 397 is serviceable enough, but not terribly satisfying. Returning to the main tune yet another time as it does seems routine and lumbering, and breaks any sense of joyful momentum toward the finish.
Another more recent idea, that of reprising the opening minore at the end, is certainly a striking effect, but is ultimately a Romantic anachronism. As much as closing with the mysterious, angst-ridden opening may appeal to our modern sensibility, in Mozart's esthetic the maggiore represents a resolution, and to undo it is to betray the sense of tonal equilibrium at the heart of his language. As with the maggiore ending of the piano concerto in the same key, so disappointingly cheerful to many of us as Chopin-besotted teenagers, we must accept it for what it is.
The following is a simple, straight-forward alternative ending. While it picks up the motivic thread, it does so in a more energetic and virtuosic way and drives to the sort of joyous conclusion toward which Mozart seems headed.
May be used freely by any who care to!
PDF: Alt ending to Mozart Dm Fantasia