So, talking about the new changes from MoM with a friend, we ended up in how prepeared spells could be maybe better for certain people or not work at all for others, to which I said "well, I like not being prepeared for certain things". This is how we reach the idea of using the short rest for something else: Once per short rest, you can change a number of prepeared spells equal to your proficiency bonus.

Of course this would not replace the hit dice or any other bonus that you get during the short rest, but it can help during long battle centered campaings and adventures to not go so blind sighted into a battle, or even to make the short rest during a dungeon a little bit more interactive and, overall, more interesting. 

To be frank, this seems like a solution in need of a problem. Spellcasters with prepared spells already have enough options to deal with a wide variety of threats without changing up their spell options that you really do not need to change your spells mid-day. Once per long rest is really sufficient for the overwhelming majority of situations, and allowing further mid-day specialisation is only going to increase the versatility of already extremely versatile classes.