This option would allow streamers to bank uncashed out bits and use them to donate to other channels, if they want. For example, one of my viewers sends me 500 bits. Those bits are banked and I can use them when making donations in the future.
This is exactly what I wanted to suggest! I believe this can help Both the streamer and Twitch. As it will take longer for bits to be cashed out.
It will also encourage viewers to buy more bits if they know their donations are going towards helping others.