Using simple deductive reasoning, The Rock vs Brock Lesnar should headline Wrestlemania 32 and nothing else (even though I profess to not caring a whole lot for it, personally.) If you want Wrestlemania 32 to be the biggest event it can possibly be, then it should be headlined by the biggest possible money match there is. And the biggest possible money match there is at the present time (reasonably speaking) is The Rock vs Brock Lesnar.
The only exceptions is if The Rock himself prefers working with someone he feels would be a much safer opponent and much less physical match, or if movie producers have put the kibosh on it on account of feeling uneasy about seeing their meal ticket get pummeled and suplexed (and possibly injured) by a 300-pound human gorilla.