Source: Wrestling Observer Newsletter (Dave Meltzer)
Based on Dave Meltzer of the WON, Roman Reigns vs. The Undertaker is the currently planned Main Event for Wrestlemania 33. Though plans can change in WWE so take it for what it is.
If Roman vs. Undertaker goes ahead as planned, it will be the sixth match in WrestleMania history to main even the biggest show without a title on the line.