The kayfabe "we're leaving if Bork loses" schtick should not be taken seriously. Regardless, he still has to serve his suspension for the PED thing. The earliest they could fight is 2018, assuming that Brock gets back into the testing pool ASAP. He also has to serve his suspension, get into fighting shape, hype the hell out of the superfight, etc.
http://deadspin.com/the-proposed-brock-lesnar-jon-jones-superfight-is-now-p-1797433212