Realistically, you may have less than a minute (

) depending on how long it takes you to play Fantaisie Impromptu.
Do you have to do something Romantic?
I really think a Scarlatti Sonata and the Chopin would make a brilliant duo, if it's short enough.
This is really tough question... sigh....
Or else you can have an all Chopin repertoire and play his "Butterfly Etude" from Op 25 (can't remember which number - this is difficult, flashy and extremely difficult.
Or Liszt's Transcendental Etude No 1 can act as a Prelude to the Chopin.
I hate it when they give a short time limit, it's almost impossible to pick a repertoire that suits...
Good luck with the competition.
Btw, what competition is this?