If the Rock wants to work, they need to make the match with Reigns happen; he has talked about it for years, wanting to put his cousin over as one of his final matches. It's the biggest drawing match to, and at Wrestlemania, they'd be maximising their financial returns given the increase in media attention for the event.
This is why the match cannot happen at Elimination Chamber. It just wouldn't have the same effect.
Punk is injured allegedly and Seth no longer has a big Night 1 opponent to.
With Cody winning the Rumble again and the need to establish him as the new cash cow, these are the viable options:
Option 1:
Wrestlemania Night 1; in my humble opinion making Gunther 'the next big thing' would be the smartest thing to do by having him absolutely crush Seth Rollins, and you have him close the night lifting the IC & WHC bit like the Ultimate Warrior at WM6.
Wrestlemania Night 2; Roman Reigns vs The Rock vs Cody Rhodes (elimination rules), Cody Rhodes pins both The Rock and Reigns; making him the biggest star in the company, all the eyeballs on him etc
Option 2:
WM Night 1: Rock vs Reigns is the biggest match possible, therefore it makes sense for it to happen, why not do this match with the winner having to defend the title against the winner of the Royal Rumble on Night 2 right?
WM Night 2: Reigns vs Cody
Either of these solutions are much better then the current proposal. They want to do Seth/Cody Night 1 and Reigns/Rock Night 2, if this indeed is the plan then I don't see HHH changing it, he doesn't have the vision of a radical promoter such as Vince who was very frustrating at times in creative, but because of his radical booking we got some of the greatest storylines ever, and he had a knack for making the next big star in wrestling; in fact Cody has his support to, which would have vetoed any animosity between Cody and HHH. With the current plan, honestly I would rather have Seth beat Cody for the WHC, when Cody wins it needs to be the no.1 guy and for the biggest prize in wrestling, but this comes with risk because anything can happen between now and Wrestlemania 41; on the flip side, by that point I think fans would be rioting if Cody didn't go over.
I must say that I really enjoyed the burial of Seth on Smackdown this week by Reigns 'You call yourself the no.1 guy after running around in your wife's clothes for a year"

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@Suleiman