You want to “freshen up” your song list, either at karaoke or with your show. I suggest you re-visit and re-learn the songs you are already doing. Take a new look at their melodies, lyrics and gooves. Chances are you’ll find a whole new song lurking in there.
Here’s a thought. If you’ve been singing some songs for a long time it probably is the time to go back and re-learn them. Treat them as if you are just working them up. Re-examine the melody, the lyrics, the rhythm patterns. If the song has a lot of words at a fast tempo, or tricky rhythm patterns “rap” them without singing. Start slow. Repeat, repeat, repeat, gradually moving up to tempo. Also, if you’ve been performing a song for a long time, chances are you’ve gotten sloppy and slurry with the lyrics. Remember to make those little words (the, and, take, to, etc.) completely understood. As time goes on we tend to emphasize the “important” words, forgetting that the little words are what make the important words important. Record yourself.
Also, if the songs are getting old to you, try new interpretations. You don’t have to make drastic changes, just freshen them up with a few new twists and turns. They just might become new and exciting again.
I read an interview with Billy Joel where he was asked how he could still make “Piano Man” so exciting and new after singing it for so many years. He said “I sing every song (including covers) as if I spent last night pacing the floor or sitting at the piano, feeling the emotions of writing it for the first time.” Keep your songs fresh, whether you sing professionally or at karaoke. Besides you’ll have a lot more fun singing your “new” songs.