It may be a bit late in the season, but here's O Come All Ye Faithful and Joy to the World interwoven, with some alternative chording that flows well. It's just me with a keyboard, singing into a Minolta camera and converting it to CD; but it will give you the idea.
I'm also attaching a chord sheet. It's unison, the melody unaltered from traditional, with the possibility of obbligato parts, but not traditional harmony, except when "O Come" asserts itself (there, four-part a capella would be great if you've got it).
Drum style may range from latin-rock to straight rock to funky-ish, or maybe some original style I haven't envisioned!
We're doing a lot of Third Day's 'Christmas Offering' collecton in their style. I personally like their version of many of the standards, but their version of "Do You Hear What I Hear" in C#minor is absolutely incredible and was extremely well received last week. We're learning about a dozen Christmas Carols/Songs for a Nursing Home visit this Friday as well as during the services.