Mario Kart 8 Deluxe benefitted from inheriting all the DLC the game already received on Wii U, plus a bunch of extra stuff added to the Switch port, plus a ton of DLC of its own, essentially nearly a decade of support in total.

If you compared base Mario Kart 8 to base Mario Kart World, they both have 32 tracks, with World also having an open world to explore. There's the distinct possibility that like MK8, World will be expanded upon over time via DLC.

With regards to Air Riders, we don't really know yet how much content it will have. Hopefully it's a lot.

I thought Sakurai showed us all the content.

The game is >20GB, plus it seems something big was teased at the end of the most recent directs. Methinks Sakurai is hiding something… though even if he’s not, the game looks like good enough value for me. I would be a little shocked, however, if a project by Masahiro Sakurai wasn’t a tad bit more ambitious.

The game is >20GB, plus it seems something big was teased at the end of the most recent directs. Methinks Sakurai is hiding something… though even if he’s not, the game looks like good enough value for me. I would be a little shocked, however, if a project by Masahiro Sakurai wasn’t a tad bit more ambitious.

I thought Sakurai showed us all the content.

As other have said, knowing Sakurai, I'm willing to bet there is way more than we have seen so far.

I think the direct showed like 4-5 tracks, that would be crazy small for a $70 game. Nintendo nowadays tends to keep a lot about their games secret at first, like how TOTK's underground map and Fuse/Ultrahand powers weren't really shown off til the last minute, or how DK Bananza's reveal trailer didn't show Pauline and only showed like 4 of its 16 layers.

Nintendo UK have randomly (maybe accidentally) uploaded 4K footage of a full race on a new track, which seems to have been cut from the second not-yet-aired Direct: