I'm going to cheat just a little bit with today's pitch. It isn't for a book but for a Doctor Who episode I'd love to see made. There's so much possible when playing with time, but I don't think I've ever seen an episode where the Doctor and companion have to solve a problem together while moving in opposite directions in time. (Granted I haven't seen every episode of the show, so it's possible it has happened before.)
The Doctor and [companion] have come to planet Etroia to visit the ancient, wise species that lives there, but when they arrive they find the planet empty and covered in mist. Upon landing they lose the Tardis and find themselves repeating the same hour over and over—but in reverse order of one another. As [companion] remembers more and more of their time there, the Doctor remembers less and less.
Now it is up to [companion] to sift through the clues the Doctor has been dropping in order to retrieve the Tardis and find the device causing their temporal misalignment. If [companion] can’t fix the misalignment before the Doctor forgets everything, they’ll both disappear into the mist, just like the former inhabitants of Etroia.