It depends a lot on what you're looking and also on what you're comfortable with. I played encores after some concertos and I usually choose a work that I like and know people will like too.
Once I played an arrangement for piano solo of a popular song that everyone knows: Bohemian Rhapsody. People loved it. Also in that place people suggested me to play arrangements of popular songs, so that could be a good point.
Other encores I have in mind for next recitals are:
Schumann/Liszt - Widmung No.1https://www.youtube.com/watch?v=IfWh4XIovMs
Rachmaninov - Prelude Op.32 No.13https://www.youtube.com/watch?v=GDX_yQ7H0Sk
Respighi - Valse Caressante from 6 pieces for piano (maybe you'll like this one)https://www.youtube.com/watch?v=T19mqDSR5ws
In the other hand, you can play any of the war-horses pieces as a Chopin Nocturne or Waltz, Rachmaninov prelude in C sharp or something like that.
If you want something more popular (also if the audience allows it, because popular music is not always well received in a serious concert - once I had an issue with the Queen encore when I told it to the organizers) and you are willing to pay sheet music, try looking trought Vika Yermolyeva's or Peter Bence's arrangements. Here some examples:
Fats Domino -The Fat Man (Vika Yermolyeva)https://www.youtube.com/watch?v=WnSnmY3gaMM
Queen - Good Old Fashioned Lover Boy (Vika Yermolyeva)https://www.youtube.com/watch?v=nNP0f0QxMFM
Extreme - More Than Words (Vika Yermolyeva)https://www.youtube.com/watch?v=R5KBk76b9HE
Queen - Don't Stop Me Now (Peter Bence)https://www.youtube.com/watch?v=LrQnnhAXLt0
If I find or remember another possibly encore, I'll comment again.