Really? That part's the one part that really comes easily to me. I'd also suggest getting out your phone or turning on the mic on your computer and singing a melody while you record. "Dee-dum-dee-doo, la-diddy-da…" Whatever syllables seem to come naturally. If you get away from looking at a piano, maybe that makes it easier to get out of looking for pattern, and to just flow. You can repeat as much as you want, but you have to keep singing, if you need time to think of what comes next, that gives you a long rest, with a pick-up, or a long note, which may be a good place for a key change. Some of it will stink. Some of it, you'll think is exactly the same as what you just did, but when you go back and listen to it, you'll find that you shifted the rhythm around, or a pitch here or there, and suddenly you've got the start of a theme and variations.
My other favorite inspiration trick is to listen to something and sing a harmony part to it that's not already there. It can be awful close to something that's already there in the music, but maybe you start singing along with the bass line, and then move to singing along with one of the inner parts, or you sing the bass line, but you make it staccato instead of legato... Again, record what you're doing, so you don't have to try and write it down while you're doing it. And then take this harmony part to Beethoven's 5th and make it into a new oboe line for your new piece. Turn off Beethoven, and write a bass line to go with your harmony, and then a melody… You'll have something that sounds NOTHING like the piece you were listening to originally, but still has some climaxes, some key changes, some direction to it, to get you out of the 4 bar repeated groove. (: