This is a reference to a Metal Gear Rising trailer, in which Raiden slices a box finding an enemy inside of it.
This is also a reference to Solid Snake, as for some stealth missions he uses a box to trick others of his location.
The location of the players that are hiding when activating the super is actually the spawning point. In addition to this, they leave a flame mark when they move.
In Raiden's reveal trailer, he said "Savor your last breath!" when activating the super, but in the final game his quote was changed to "Playtime is over!". That original quote is now used when the player successfully score kills.
The green watermelons you hit if you don't slice a player is a reference to Metal Gear Solid 4, on how Solid Snake tricked a Gekko to step on a cardboard box they thought he was hiding on when he was actually someplace else, making "The Box Revengeance!" the only level three where you must actually find your opponents, because you do not know where they are for certain.
It is also a reference to one of the first Metal Gear Rising trailers, where Raiden is seen slicing watermelons.
When Raiden finds an opponent during the Super, the classic Alert (!) sound effect from the Metal Gear series will play. It is also present in the Franzea stage when Metal Gear RAY attacks the players.