#2 A huge return

At WrestleMania this year it was announced that Rey Mysterio would take on Samoa Joe for the US Title, but after Rey was injured in his match against Baron Corbin, many are doubtful whether the legendary luchador will compete or not.
Rumors that John Cena could compete at WrestleMania this year incited speculation that Rey's apparent injury could be WWE's strategy to get Cena on the show, and that would be the perfect way to open WrestleMania.
Female superstar says she inspired John Cena turning heel HERE
Samoa Joe vs John Cena is a dream match that many are clamoring to see, and if WWE intends on giving us that match at the event, it would be best suited at the start of the show.
#3 A son's revenge

When Shane McMahon destroyed the Miz and his father at ringside after they failed in their quest to win the SmackDown Live Tag Team Titles, fans knew that this was going to be a heated match at WrestleMania.
McMahon has put on some truly memorable performances at WrestleMania, as his legendary war with his father at WrestleMania 17, his insane dive off the cage at WrestleMania 32, and his excellent opener with AJ Styles at WrestleMania 33 are some of his best contributions.
Miz has grown into a fantastic wrestler over the last few years, and placing these two competitors at the start of the show in a falls count anywhere match would get fans pumped up for the rest of the event.