#4 Something new for John Cena and Nikki Bella
Of the dozen matches John Cena has had at WrestleMania, all of them have either been one-on-one or Triple Threat matches. Additionally, each of them (with the exception of two have been contested for championships. This is the first tag team match he will have in 12 WrestleMania appearances.
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As cool as Cena vs. The Undertaker would have been, it is somewhat refreshing to have Cena in a match at 'Mania that isn't anywhere near being the main event. It's a midcard match at best, and as a result, it's a refreshing change of pace for him.
Meanwhile, Nikki Bella has really rejuvenated her career since returning to the ring last summer, but having her go after (and likely win) the SmackDown Women's Championship at WrestleMania would have been way too predictable.
She and Cena have been dating for close to five years, so it's great they're finally able to come together for a televised program in time for WrestleMania.