Well, Jon Jones called out Lesnar tonight after he won his fight, so I guess this goes from a rumor to a possibility.
Thing is, if Lesnar really is interested in fighting Jones, he is going to have to resubmit to USDA testing, and finish out his suspension very soon. They're saying that UFC would want this fight to happen in December, and for that to happen, Lesnar would have to re-enter the testing pool in the next couple of weeks, and he'd probably need at least 8-10 weeks of serious training in order to be ready to fight Jones - which means no more WWE after September. Which means supersonic is right, no Lesnar at Survivor Series, if WWE would allow that. We don't know the conditions of his WWE contract, of course.
It would also mean that as far as I'm concerned, Jones vs. Lesnar might go down in history as the biggest fight between two people who appear to be genuinely unpleasant human beings. I'm not sure who I'd pull for in that fight, I don't care for either guy one bit. Kind of reminds me of Nate Diaz vs. Conor McGreggor.