it's super unlikely they'll go any bigger. maybe 65" to conform with the competition, but getting anything larger would probably double the price. are you willing to spend 6k for a 70" f8500?
if you want that 'theatre' experience 70" isn't gonna cut it anyway, you have to go front projection if you really want a home theatre. otherwise you might as well just sit 2 feet closer and pretend it's a huge screen, haha. the f8500 is so clear, you probably could do that actually. there's almost no pixel structure visible
as for the F8500 being an upgrade, yes, yes it will be. if the f8500 was released in 2012 it would have been the runaway winner of the best tv produced. the only reason it wasn't this year is because panasonic also made some huge improvements in their line. 2013 was a GREAT year for plasma and made many ppl finally replace their 'small' kuro's for a larger set. the f8500 and vt/zt were 'close enough' for many to enjoy the extra inches