Pixel Plains is a course in Super Mario 64 Deluxe Edition. It resembles 1-3 from Super Mario 3D Land, with few differences. As the only footage of this level exists in a now-deleted post by Nintendo, not much is known about this level. It presumably has seven missions.
Geography[]
Pixel Plains remains an exact copy of World 1-3 from Super Mario 3D Land, except the pit beneath the player has been replaced with a grassy plain. A MIPS is on this plain, and catching him rewards the player with a Power Star.
If the player continues onwards, the player will see that the Switchboards (existent in 1-3) have been removed, and replaced with a stone pathway. Presumably, the rest of the level remains similar to 1-3, with minimal differences. No further footage of this course was shown throughout the trailer.
All enemies unique to Super Mario 3D Land have presumably been replaced with Goombas and Chuckyas, as no unique enemies were shown during the trailer. Warp Boxes, existent in Super Mario 3D Land, were not shown in the trailer, and may have been removed. New pathways between level sections were shown, possibly as an alternative to Warp Boxes. The Coin Heaven was not shown, so it is not known whether or not it exists in the game.
Missions[]
Only two missions were showcased in the trailer. A mission involving catching MIPS, and a boss fight against Eyerok. There were most likely more missions, although they weren't shown. Notably, Eyerok's eyes are colored red, instead of their usual blue, during the trailer.
Theorizing[]
This level may have been an early version of 1-3 from Super Mario 3D Land, or a side-effect of the "Customization" option shown in the trailer. Not much is known about the latter, so it's difficult to say for sure.