#2 Brock Lesnar and Roman Reigns for the Universal Title

While this would involve putting The Undertaker in the Universal Title match between Roman Reigns and Brock Lesnar, which is rumoured to be the pay-per-view's main event, it could have an amazing payoff for Roman Reigns if he was able to defeat two of the company's biggest stars in one high stakes main event.
With that being said and The Undertaker supposedly in good health, it makes sense for The him to appear at the 25th anniversary of Monday Night Raw in January and confront Roman Reigns. Furthermore, adding him to the Universal title match also gives Taker one more chance to avenge his two losses at Mania, which would be a huge selling point on its own.
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In the end, it might be a little bit too much for WWE to be able to put together in only a few months, and could be too gimmicky for fans to follow anyway. Sure, fans would be excited to see The Undertaker have one more chance at defeating the two men that ended his streak, but once fans realise its all to get Reigns over, they probably won't be behind it.
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