With Sonic Forces, fittingly enough. Both were released in the fourth quarter of the year (within a week of each other, even: Odyssey was released on October 27 worldwide, and Forces was released on November 7 in the US and November 9 overseas), and both aimed to take their respective series in a bold new direction. Odyssey ended up being much-better received than Forces in the long run.
Also with A Hat in Time, being another classic-styled 3D Collectathon platformer released in the same month, with its console version launching first on rival systems.
In a rematch of the battle between Super Mario 3D World and Knack — which surprisingly resulted in Knack upsetting the odds and just barely outselling 3D World for a scant few months in the UK, albeit with the worldwide sales of the latter game (5.75 million) ultimately demolishing those of the former (1 million+) in the long run despite being on Nintendo's worst selling home console — this game was released a month or so after Knack II. This time, the result of the duel wasn't even close, with some estimates putting this game at selling twenty times the lifetime sales of Knack II in its opening weekend alone.
Being a return to exploration-based, collectathon platforming was a thing a lot of people guessed. Initial teaser footage was very vague on whether this was the case, or if it was going to be another game in the vein of Super Mario Galaxy, 3D Land, or 3D World.
Pauline being part of New Donk City. Keen-eyed viewers noticed a few references to the Donkey Kong games in the background, and thought she'd fit the theme.
Many people were expecting Crazy Caps to play an important role in the game, given that hats appeared to be a common theme throughout the trailers.
It took no one by surprise that the mushroom-shaped landmass on the map was the Mushroom Kingdom itself.
No one was surprised by the moon being the last level, as it was strangely focused on during trailers despite being just a background detail.
Bowser getting Captured by Mario in the finale was universally expected as soon as the Capture mechanic was first revealed at E3 2017.
Many people saw a kingdom based on Japan coming. There was plenty of old promotional artwork of Mario in traditional Japanese outfits for the game to draw from, and where else would they be found but a Japan analog? It being Bowser's Kingdom, however, took everyone by surprise.
Luigi being part of an update. Fans were guessing that the infamous castle courtyard from Super Mario 64 being in the Mushroom Kingdom was hinting at something pertaining to him.
In the months leading up to the game's release, many fans theorized that New Donk City was the setting of the original Donkey Kong (and the original Mario Bros.), that ended up being exactly the case.
The fact that there are realistic humans in the game that contrast heavily to the short, cartoony Mario led many to theorize that Mario wasn't human at all, just humanoid (helped by supplementary material released long before this game, apparently, claiming that Mario's race as Homo Nintendonus, not Homo Sapiens). But no, that's not the case, a post E3 2017 interview would confirm that Mario was indeed human. As a matter of fact, the inhabitants of New Donk City are never even called "human," they're "New Donkers".
A large number of fans were convinced that the game would end with a Relationship Upgrade for Mario and Peach, with them either getting engaged, or marrying. While Mario does try to propose, the game preserves the status quo.
Killer App: Naturally. The game sold 2 million copies during its first weekend, and ended up selling over 9 million copies in just two months. Meaning that over 60% of all Switch owners have the game.
Spoiled by the Merchandise: The boxes for the amiibo figures of Mario, Peach and Bowser (in wedding outfits) ended up giving away the existence of the Lake Kingdom, Snow Kingdom, and Mushroom Kingdom, thanks to the giant world map behind the figures.
What Could Have Been: The book The Art of Super Mario Odyssey reveals concept art for several unused elements in the game. These include, but are not limited to...
Different outfits for Mario.
New Donk City not based on the Donkey Kong series but rather Mario-themed.
Bowser having his own Capture hat, complete with concept art where Peach gets turned into a fusion of herself and Bowser thanks to it.note This was made way before the Bowsette meme popped up after the September 2018 Nintendo Direct.
Working Title: The "Steam Gardens" area was originally known as Kogwald. The rename was likely made to better match the locale, since cogs are mostly absent from its steam-based machinery.
This is the first mainline Super Mario game to get above an E rating by the ESRB - E10+, to be precise.
In Japan, it's the first mainline Super Mario game to get a B rating (for ages 12 and up).
In Europe, it's the first mainline Super Mario game (excluding re-releases) to get a rating higher than PEGI 3, PEGI 7 to be exact.note The Wii U re-releases of Super Mario 64 DS and New Super Mario Bros. shifted from PEGI 3 to PEGI 12, due to changed views on gambling minigames.
This is the first mainline Super Mario game in years since Super Mario Sunshine to feature actual voice acted dialogue, although in most cases it's gibberish with hints of the character's names thrown in, similar to many video games of The '90s. Mario and Peach are arguably the only ones with noticeable actual words besides Pauline and the announcer in the New Donk City festival event.