Well, according to my opinion Fuji will try to make as much PROFIT as it is possible AND to see wehter enuogh people would be potentialy interested for another folder (wider/larger/more fancy - you name it). The 6x7 is probably a good compromise between the size of the camera and the size of the negative. a 6x9 would be larger and heavier (to keep the film flat - I guess). Multi format ability means more $$$ as well. I just hope that Fuji comes out with a nice sturdy camera with buit-in lightmeter (preferably TTL), AE mode, good lens, a few accessories and not completely ridiculous price.
Personaly - I would prefere somewhat more prolongated format - like 6x8, with mild wide angle lens (like 35mm on small format), but would still hope that the camera would weight less than 1 kg and cost less than $1500.