Brian May's guitar sound is too important to most music they could have performed in concert so I couldn't see an acoustic concert being feasible.
On the other hand some songs would have been amazing with Freddie on piano accompanied by a String Quartet. Love of My Life and You Take My Breath Away come to mind.
Brian did do that acoustic special on VH1, so it might not be beyond the realms of possibility, although some things worked better than others on that show. Personally, I think Brians electric guitar sound is too integral to the Queen sound to be dropped completely. You can tell it's him playing an electric within seconds, but I would be hard pushed to pick him out playing an acoustic.
I did initiate a similar topic sometime ago....here's the link.... link
....but to preface, some of the suggestions for an unplugged gig were....
Who Needs You?
Innuendo
Calling All Girls
Melancholy Blues
Love of My Life
Is This The World We Created?
There were of course the usual "Queen-can't-do-acoustic" nay-sayers. That's just nonsense.
Cheers
Paul