#1 It will be the most financially successful WrestleMania of all time

The Rock, Shawn Michaels, The Undertaker, Ronda Rousey, John Cena, Batista, Brock Lesnar, Cris Cyborg- all of these names have been linked to an appearance at next year's WrestleMania and if the WWE can manage to get at least 60% of them to appear on the show, then they can expect a very profitable return indeed.
Cody Rhodes' uncle is a WWE Hall of Famer. More details HERE.
With the age of buyrates long gone thanks to the rise of the WWE network, it has become slighter harder to judge what makes for a successful PPV these days. WrestleMania 34 made a whopping $14.2 million for the Mercedez-Benz Superdome Stadium earlier this year, becoming the most successful sports entertainment event in the stadium's history though, which makes for a pretty strong litmus test.
It seems highly unlikely, given the talent that is linked to next year's event, that next year's WrestleMania won't go on to top that. It will also probably give network subscriptions a massive boost, which, of course, is seen as a key indicator of success in the WWE these days.