Just about any revue will work with just a piano. THE FANTASTICKS was indeed written for piano and harp, but you can get by with just a piano. If you're feeling adventuresome, there are other things in the Jones/Schmidt canon that are pretty much piano only: BONES, CELEBRATION (a sadly neglected work, IMHO), ROADHOUSE, and of course I DO I DO. If you're willing to do a little research, there's a revue based on the cartoons of Jules Feiffer that's also in sore need of revisiting, particularly these days (somehow, those things he wrote about in the 50s and 60s are back all in their obsessively crazy wonder). I LOVE YOU YOU'RE PERFECT is a cute show but falls apart in the middle of the second act; still, it would work.
Insofar as scaled back works, if it's done intelligently and with concept, it can be done. SWEENEY pretty much hit the mark. There was a minimalist MY FAIR LADY in Chicago that was incredibly well done, and the small-scale revival of PACIFIC OVERTURES was a tiny gem. Like everything else, it depends on how you approach it.
Now, is it fair to someone who's spending a hundred-plus bucks on a ticket? IMHO, no. But that's another issue altogether.
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