#2. Creating buzz for WrestleMania

As if this WrestleMania didn't need enough buzz, WWE will take a step extra. Having the match between two legends will definitely help create some buzz among the casual audience and older audiences who would return to watch WrestleMania.
It's a great way to go about it, especially given the name value that Batista now holds. It would be surprising to have two retirement matches in one night, but crazier things have happened in WWE!
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#1. A match for the mainstream audience

Fans need to realize that WrestleMania is appealing to a much larger crowd than the vocal internet fanbase. As mentioned, Batista has a high-name value and can bring in some mainstream attention, the kind that WWE so desperately craves for.
Having it be a retirement match will only further add interest because fans would want to know what Batista's status in WWE and wrestling will ultimately be.
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