An idea for those thinking about implementing Etc1 compression because they noticed the 3DS supports it

[LexiBigCheese]

In case you don't want to look around too far: https://github.com/Cruel/3dstex/blob/master/src/rg_etc1.cpp
I would also recommend looking into how the textures are swizzled, but note that the swizzle_3ds crate does not currently support swizzling those textures, so you'll also have to reference 3dstex for that.
I also implore you to support not only encoding, but decoding, as well as encoding/decoding only a part of the texture, to allow for patching the texture (decode the portion that needs to be patched, patch within that portion, then encode the result back to the rectangle. Then the user can swizzle and upload, though it may be an idea to encode/decode while keeping data in swizzled form.)