One of the reasons I’ve never gotten much into digital comics is that while on the surface they seem convenient — no need to go hunt for an issue in a box — they’re terribly hard to re-read if the series has any crossovers, annuals, reboots, etc. A book like Saga, you just read straight through from 1. Star Wars and Darth Vader, on the other hand, have interrelated storylines. A character leaves one book and shows up in another. Luke encounters someone in one series and then Darth Vader is informed of the incident in his own book.
Ideally, then, a comics reading app would let you sort your “up next” list accordingly, perhaps by release date, with a way to manually override the sort order. Maybe even a way for me to set up a reading order and then share it.
But all the mainstream books are copy-protected, so you’re stuck with whatever their parent app does, and no one can try to build their own reader app.