Cadenza Ideas?

Hello

I'm writing a piano concerto and I have got to the cadenza. I am stuck with what to do with it. Do they have certain distinguishing features and do they have to be in the tonic key?

In the old days it was improvisatory. It wasn't until Beethoven really, where the cadenza was expressly written out.
I rewrote the Haydn Cello Concerto in C major's cadenza in it when I performed it to something extremely contemporary and different. No real rules here, although it might be nice to hear some of the motifs from the piece worked into it.

Thanks @Monarcheon