Retina display - iPad 3 or 4
Optically Laminated Display - iPad 2
2 cameras
- iPad 2: 1mp, with 720p and VGA
- iPad 3: 5mp, with 720p and VGA
- iPad 4: 8mp, with 1080i / p and 720p front facing camera.
512mb RAM - iPad 2
1gb ram - iPad 3
New design - iPad 2: Like iPod Touch 4
- iPad 3: Possibly Liquid Metal
New speaker - iPad 2
Sd slot - iPad 2 / 3
Mini/micro USB - iPad 3 / 4
3D Display - iPad 2, if Apple want to be the first (depends on how much optimus pad costs and if it'll release before April). iPad 4 if they want to wait for costs to go down and improve it (viewing angles).
Higher Contrast - iPad 2
A5 (based on A9 dual core) - iPad 2
A6 (based on A15 dual core) - iPad 3
A7 (based on A15 quad, or possible Tegra 4, but I doubt it.) - iPad 4
Tegra 2 standard graphics - iPad 2 / 3
Tegra 3 standard graphics - iPad 3 / 4
I don't think that Apple will change to the tegra chipsets, from the ARM processors; however, the Tegra 2 supports 3D, and a 3D tablet will be unique and will beat off Xoom, Galaxy Tab etc, (as long as it has a 5mp camera), and will have little competition, apart from the Optimus pad, which is rumoured to have a 3D display.