He's coming back to put Roman Reigns over. This will be the ultimate "passing of the torch" for Reigns. Reigns is seen as the beast, animal, if you will, who better to put him over than the animal himself? That would be a massive Mania moment, Reigns pinning Batista.
Reigns? A beast? Well, he is beastly in the ring. In this tense though, beastly refers to how bad he is, not how you intend it to mean as strong or dominant.
Reigns pinning Batista is barely a blip on the radar of a house show, let alone a massive Mania Moment.
To answer the question: How will they book Batista? Answer: The same way they book every returning star except Taker: BADLY. Batista is only coming back to pull a Rock like movie promotion for the big movie he's in that is supposed to be out next year "Guardians of the Galaxy." That's it. He's in if for the promotional work and the payday a few appearances will net him. He 's not back because he loves wrestling, he's not back because he respects wrestling fans. He wants the money and to spread word about his movie and that's it.
If he is around for a Mania moment, then the most logical place to go would be Batista vs Lesnar. Batista was basically pushed as the replacement to Lesnar when Brock left to try football and UFC, and when he left, he tried to follow Brock into MMA with far less success. There's of course one other name that would complete the MMA trifecta. Bobby Lashley.
This I would pay to see: an MMA shoot style triple threat cage match between Lesnar/ Lashley/ Batista. This or some kind of match featuring 2 of those three are the only way I would pay money to see any of them compete in the ring again. Not even any or all of the three vs Taker or some kind of tag involving the 4 would get me to pay out for a poor quality Mania lineup.
Batista is out of touch with the wrestling fans as he's ignored wrestling altogether since leaving in what 2006? 2008? So he's at least 5 years removed from any relevance, let alone prominence in the sport. He failed as an MMA performer and fell into playing back up roles in movies, mostly low or mid range quality, and the only reason his name is being elevated again is that he landed a role in "Guardians of the Galaxy." He's below mediocre in the ring, is dull as a turd on the mic and couldn't tell a good story with his in ring work if you put a gun to his head.
Oh, wait. That does sound like Reigns.