This is an interesting topic. I like you assumed that if Rock returned it was going to be against RR at mania. However Rumor has it, that there is big money on the table for Rock vs. Reigns in Australia. Rock shouldn't win the belt, and he will not, but I think Cody and RR match up again at WM and Cody gets the belt. All that said, they do have an unhealthy fascination with length of reign. Who care, it is all scripted.
I didn't realize Elimination Chamber was going to be in Australia. Haven't been following closely at all. I'm not even really sure how the $$ works. I know for the Saudi shows, the Prince just says who he wants to see and cuts WWE a fat ass check. Storyline means nothing. But EC isn't a glorified house show. Who is paying for certain guys to attend from a democratic country? I feel like guys like the Rock don't return unless it's for WM. Much less a mid-PPV. Austin's return to fight KO wasn't at like MITB or Payback or something.
We also have to see who wins the Rumble still. And do they choose Reigns or Rollins at WM.
I could see something like Rock fights Cody at EC. We get interference from the bloodline. Reigns has to defend on both nights of WM. Rock on night 1, who is the real head of the table? Reigns wins. Cody on night 2 against a beaten-up Reigns. Maybe the Rock interferes, could aid Cody because of the Reigns feud. Could aid Reigns because "family". Rock could just take a bow after night 1 and call it a day. Assuming the Rumble winner choses Seth.
I think a Rock/Reigns throw away match at EC wouldn't give a legend the respect he deserves.