In regards to the bg colors, it would be nice to have a custom rgb/hex option for people who dont want to key out the bg and are cool with having a solid color there.
For the gif support I might recommend that they are somehow synced to the same timing even when not visible as in in another emote or mouth shape. In other apps I have used gif files to have blinks with multiple frames/my own custom timings and to syncronise them I made sure every asset that had the eyes in it were the exact same duration and then when they changed between gifs they would always be at the same stage of a blink, so I think having them keep track of frames in the background is important.