Press Kit Assets give a peek into some future synergy. UPDATE: They're already available!
Super Mario Run came out in December last year, but future updates will add content to the game to match this year's upcoming Super Mario Odyssey. This wasn't announced directly via Nintendo's E3 Spotlight presentation, but official assets show Nintendo's plans to add new statues in the game featuring "captured" enemies from the game controlled by Mario's "Cappy" sidekick hat-character.
There's no news yet on when the crossover will be released, but it'll likely be close to Super Mario Odyssey's October release date.
UPDATE: The crossover items are now available in Super Mario Run, provided players update their game with the mandatory download. The items come in the form of buildings you can place in your kingdom that are available for a limited time and can be purchased for coins.