I say "fully" because the Generators already do support transparency in the preview. But when the creation is actually generated, the transparency is gone.
HOW TO IMPLEMENT MY IDEA:
1. Memes
Supporting transparency in memes is a very simple task. Imgflip merely has to support PNG memes.
2. GIFs (video)
This is the hardest of them all. This is the only generator which does not support transparency at all.
To do this, Imgflip needs to support conversion to other video file formats which support transparency.
3. GIFs (image)
Like memes, it is simple. To do this, Imgflip has to change the code a bit to use the <img> tag, as the GIF file format, which supports transparency, is an image file format and not a video one.