While LEGO very rarely expands Ideas sets into full-blown themes, the ability to connect it to other sets does seem to suggest they may have further plans for Sonic. What hasn’t changed is that the set still appears to be modular, per the pins sticking out on the right side. This change makes it look a bit awkward, but it’s always possible LEGO doesn’t have a great process to print his nose on a 2×1 plate with handle bars like in the original design. It uses plates sticking out at diagonals with wedges to make his mustache, and a nose sticking out front. Eggman’s body looks mostly unchanged, though his face has been altered in kind of a weird way.It also seems to have a secondary face to make it look angry. Motobug’s face is rather similar to the original design, but its body is now bigger with more round components and details.He also appears to have a more traditional bright blue, instead of the slightly muted blue SEGA uses to differentiate classic and modern Sonic. He has full-sized legs instead of the Toastergrl design’s short legs. Sonic’s minifig Looks much more like his LEGO Dimensions version, but without the green eyes.The sunflower has been redesigned with a little more detail.The back of the box seems to imply there’s still a mechanism to make it bounce though. The water underneath is now made of transparent bricks. The log design is made of 1×1 round plates. Instead of using pin-connected technic pieces, the bridge is now much more flat and solid.The places where the grass transitions into the walls now do so using specially printed 1×4 tiles and quarter-circle tiles.The item boxes are slightly chunkier now, using 2×2 tiles to show the screen instead of printing it on 1×2 bricks.It also appears that the tall sides of the loop are either printed or stickered pieces. The Ideas design builds each square 2-bricks wide, but the retail set reduces it to 1-brick wide (and possibly made of 1×1 tiles?).
The Phantom Ruby is gone, but you do get a full set of Chaos Emeralds.There are, however, floating rings, the Eggman hovercraft, a Crabmeat, and a checkpoint post.There are no longer any Hard Boiled Heavy figures, flickys or the giant Eggman Robot.You can even see a Lives icon in the bottom left of the model. Toastergrl’s design has slight dimensionality to it, but the retail design emulates the 2D level design of classic Sonic games.
Toastergrl’s design touts about 700 pieces, while the box states 1125 pieces.
Often times, it is to follow the license holder’s suggestions or requirements, to fit within what is reasonable for LEGO to manufacture, or to ensure the design can be made and sold at a reasonable price. It’s quite common for a set to go through significant revisions between the Ideas version and the final released version. You probably noticed right away that the design has changed a fair bit from what was originally pitched on LEGO Ideas: UC0UV0rpu6- Jordan Middler December 24, 2021