I mean, he perked right up and started half-shouting about this: “Roughly one month ago, data miners found some code suggesting that Super Smash Bros. Ultimate was getting the Stage Builder mode added. This addition would most likely be part of the 3.0 update that will bring Joker into the fight. Well, now it looks like Nintendo has accidentally leaked that Stage Builder is in fact coming to the game.” My eldest told me that this means right what it says on the label: with it, he’ll be able to design his own stages for Super Smash Bros fights.
Also, he already knew about the rumor, which surprises me. I don’t know why it should, though. He’s clearly more invested in this game than I am.