Heard rumors the 717s are coming back under the QF banner from Cobham.
Correct
however it won't be the 717's...they are being scrapped as of 2017. An as yet to be decided new type will replace them. Back under the mainline banner, flown by mainline pilots.
What is stopping another Australian Airlines 767 longhaul with new equipment ?
Nothing. It just can't be called 'QANTAS' or painted as a QANTAS aircraft. The only reason Jetconnect get away with it is because NZ falls outside the FWA jurisdiction.