1. Dash Ramps on Walls and Ceilings: This has those red dash ramps on walls and ceilings, and they are usually at their edge and can point in different directions. Either way, this could help with getting to certain routes easier.
2. Brake Jump: When Sonic does a brake to slow down, the moment he does, so, the player can then press a button make Sonic jump higher than normal, to reach higher places.
3. Skid Jump: This ties in to another idea of mine from earlier. When Sonic skids sideways while facing a perpendicular direction when running, when the player presses a button, he will jump in that direction really far. Watch from 0:08 here.
A demonstration of Tiara B's gameplay loop for Sonic X-treme. Converted to MP4 video from Chris Senn's original GIF animations.
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It is kind of like that. It can be used in 3D or 2D. This can make platforming in perpendicular directions much easier.
4. Diagonal Plane Switching. In some areas in 2D places, Sonic can take ramps that extend diagonally, either by doing a sharp turn (normally with the Drift) or merging onto the diagonal ramp by pressing up or down when running. Sonic can not only reach other 2D planes to run on, but depending on his speed, he can either reach areas in between or fall in bottomless pits.
5. Accumulative Spin Dash: Look at the beginning of here.
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Notice how Sonic Spin Dashes to become a giant snowball. Well, with a Sonic Adventure styled upgrade, Sonic can do this on any terrain or collection of items that are nearby, allowing him to take on the form of whatever is near Sonic in spherical form. Just like with Namco Bandai's Katamari games, Sonic can roll with these items around him, acting as a bowling ball that can destroy tough targets.
6. Different Starting Points: Look at this.
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At the beginning, the playable character is at a different starting from the one at 3:49, which is the usual starting point of the level. With this, Sonic can find means, like an alternate goal, a hidden spring or something else, usually near the end of the Act, that will lead him to different starting points and new sections of 2D or 3D levels. Heck! In the boss act, Sonic might fight a different version of the boss through these methods!
7: Back and Forth 2D. Now look at this at 4:23 to 6:09.
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Notice the curving paths and changes in directions. Well, this idea would work like that but with the classic camera, meaning no changing camera angles, and Sonic would go back and forth on whatever curvy or zigzag-like path he is on and more, looking like this at 0:32 to 0:34.
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To me, this would make for a really nice 3D effect.
8. Reduced Angles: Now, look at this
This, along with other round shapes, would be harder to run around properly in 3D, but what if there were lines Sonic could run on while moving on these shapes, like the one on the sphere in the image? There would be a number of line which Sonic can move to by Quick Stepping onto them. Sonic can still run on these structures without the lines, but by running on these, Sonic can then more easily jump off off of them or onto nearby roads to alternate routes. These would be hard to get onto without the lines.