two cameras come to mind, allthough they ar not too easy to find:
Rittreck! here you can have a multi format cassette, that allows you to chose between 4.5x6 to 6x6 to 6x7 in the same cassette - and you can actually change format during one film.. if that makes sense..
then you can have a 6x9 cassette also.
interchangeble lenses (Muhashino, as far as I remember) or just buy some nice old lenses...(Dagor etc)
http://www.emilschildt.com/EGO-KAMERAER - Rittreck.htm
MAKIFLEX (Plaubel) huge camera with negative sizes as the Rittreck, but bigger, as you can turn the back, the Mamiya way..
extra plus: it is possible to put 4x5" cassettes on the back!! then you'll have a fantastic 9x9 cm negative with round corners!! (as far as I know, the only camera doing this).
also fit for Polaroid back, with allmost full negative.
interchangeble lenses.
as said: huge (and ugly), but a fantastic tool!!
I don't have pictures (yet) of the MAKIFLEX on my homepage, but I can send you some if you're interested!
some from google:
http://images.google.co.uk/images?hl=en&q=makiflex&um=1&ie=UTF-8&sa=N&tab=wi