These are some modifications that I have done to a WitEden 2x2x2, and I am not aware of other mods existing for this cube.
DSCN0018.JPG [ 123.72 KiB | Viewed 272 times ]
The Shuang Ren core is the best design that I've seen, so I used it.
DSCN0022.JPG [ 122.99 KiB | Viewed 272 times ]
This inner edge mod keeps the cube from locking up from internal pops on looser tensions.
DSCN0027.JPG [ 124.99 KiB | Viewed 272 times ]
I split the tabbed edges in half, and glued them to the centerpieces. I then filleted them where it was needed.
DSCN0028.JPG [ 132.88 KiB | Viewed 272 times ]
This is my core assembly, with the moveable centers having 2 washers with rounded sides facing each other, and the bandaged centers having no washers. The screws are from C4U. I pressed nylon washers on the center stalks so that the pieces don't sink into the puzzle. They also reduce internal pops.