I went to a church some years ago, and they attempted to try a new song they wrote. It sounded great in rehearsal, but the members just kind of had a blank stare, Prob, because they didn't know the words.
I think if I do attempt on my own, it would be something simple that everyone can sing along too or pick up really easy.
Sometimes when the spirit is moving and we're playing instrumentals, the worship singer would just make up something on the spot.