As I think this belongs back on top... (warning for m1469, solution follows)
After figuring out that this piece got to be a fantasy in the key of C, with at least a part of it in c-minor, and knowing that it isn't the Wanderer, and after looking at Schubert's other Fantasy, which is also in C (and surprisingly close to the Mozart...) - the Schumann Fantasy op. 17 was the only one left.
So I looked at the sheets, and indeed... deep down (2nd page) in the second movement, as a little melody, well hidden, it was there....(
https://www.sheetmusicarchive.net/compositions_b/schm17_2.pdf)
That was a bit tough in my opinion (that is, without the other information). Not because the piece was obscure or something - it is just that you wouldn't really associate this melody with the Fantasy, would you?
Still fun though.