Im currently playing a 16th Level Druid in my game and consequently have a very large number of spells to choose from. We have been jumping around a lot and have just recently decided to face some vampires, so i changed my list to have all the water spells i could find (as it is a way to kill them) However mid session plans changed and we had to do something else as a Big Bad decided to launch an attack against us. We had the chance to have a long rest in which time i had the opportunity to change my prepared list but i found i could not remember all the spells that were in my normal combat prepared list.

So i think it would be really cool to be able to save lists of prepared spells that you can quickly flick between, or give the option to change to a new one when taking a long rest, its probably more of an niche request but it would be really helpful in some situations. Especially when you are changing quickly and forget an important spell. This has append to me before and as a result we now have Uncle Sam posters up around the our DnD room saying "We want you, to prepare Planeshift".