Of those two choices, I'd prefer to hear The Mac cover someone else's tune.
If another artist recorded an *original* composition... such as Sheryl Crow doing "You're Not The One," or the Williams Brothers doing "You Like Me"... then I'd think it was cool.
But if it's just a cover of an already released tune, then I wouldn't be as interested, since I always tend to prefer Fleetwood Mac's original version.
So The Mac covering someone else's song would definitely be preferable.
I'd really dig it if they did a cover of "Happy Together," since that's in my top 5 of all-time favorite songs.
I also think it would be cool for them to tackle "California Dreaming," given its role in the Lindsey & Stevie Saga.
And I wouldn't mind if "She Smiled Sweetly" turned up on the new album... but mostly I would much rather have completely original compositions.
Interesting question, John!
Johnny Stew