UPDATE 22/2/18: Super Mario Odyssey's free update is now available, adding a new mini-game, two snapshot filters (Coin and Neon) and three outfits as promised.
Provided you've finished the story, Luigi will show up in each world (use your map if you can't spot him) with the offer of finding or hiding balloons with other real world players.
Successful attempts give you Coins as a reward, with rankings and win streaks that give you even more earnings, possibly making it a nice earner as you grind for those final pricey costumes from the Crazy Cap store.
Nintendo's penchant for friend codes means you can also directly challenge others with a particularly well-hidden balloon if you fancy.
The outfits are also locked behind completing the game, and aren't cheap - the Super Mario Sunshine set is 4500 coins, for example. Though to be fair, that's a small price to pay to pretend you're playing the secret best 3D Mario game.
ORIGINAL STORY 11/1/18: Super Mario Odyssey will get a free update in February with a new mini-game to play after you've finished the main story.
Luigi's Balloon World sees you hiding or searching for balloons within the game's existing levels, with leaderboards attached so you can see how you match up with others.
When searching for balloons you'll be looking for those hidden by others around the world - so good luck tracking down all the hidden spots you'll likely have missed.
In addition to that, there will be three new outfits - including, amazingly, Mario's shades and shirt from Super Mario Sunshine. A musician suit and suit of armour are also included.
There will be a few new snapshot filters too.
Watch the update in action from around the 6:30 mark above.