Paul Heyman announced in RAW of this week that there’s no one on RAW, SmackDown or NXT who’s deserving of facing Lesnar for the WWE Championship, so Lesnar will enter the Men’s Royal Rumble 2020 match as the #1 entrant.
According to WrestleVotes, WWE add Lesnar in the Rumble match to begin his feud for his match at WrestleMania 36, where his opponent could be from SmackDown or NXT:
“WWE is doing this Lesnar in the Rumble angle as a way to set up Brock’s Mania matchup. Which could signal it’s against someone from SmackDown or NXT as Heyman mentioned a few times last night. Opens up a world of possibilities.”