I’d love to see the Inverness to Aberdeen line covered. For a start, it’s my “local” rail link, but it also has the advantage of having only eight stops along the way at which all trains stop. You mentioned the old and abandoned lines, and there are several that are walkable for certain distances (Elgin-Lossiemouth and parts of the Hopeman Branch, for example), or Keith to Dufftown, where you can actually get on a train on certain days of the week. The only snag I can see is that some stops will have many more things to see than can reasonably done in a day!
Whichever route you do choose to write about, I’m certainly looking forward to the next book.