I would propose:
On-topic requests will ask for an ordering according a well-defined, non-subjective criteria.
Here I'm thinking of things like:
- Publication order: "In what order were the Narnia books published?"
- Order by the internal chronology within the fiction: "What is the chronological order of the Narnia books?"
- Writing order: "Jane Austen's books were not published in the order they were written; in what order did Austen originally write them?"
- Identification and ordering of sub-series:"Which Discworld books have the Witches in them?", or "What are the various sub-series of the Discworld books?").
For large, intricate series, I think requesting a reading guide to a series makes for a great question -- based on chronology, subseries, a common element ("What are all the Dragonlance stories with Raistlin in them, by the fictional chronology?"). As long as a well-defined criteria for the guide is given, I think this falls within the bounds I've described here.
There might be other well-defined orderings that could be asked about; this can't be an exhaustive list.
But I would like to see reading-order questions nudged towards asking for a particular order - publishing, chronological, etc, rather than just asking "What order should I read this in?". I think it would be much clearer to have one question for publication order and a separate one for chronological order, than to have those be two different, contrasting answers to the same question, with vote-counts implying that one of these is "better" or "more correct" than the other.
Which is one part of a larger observation: requesting subjective suggestions for reading order, is exactly the same as asking for a reading recommendation. It may be a limited, well-scoped, well-defined reading recommendation, but it still boils down to "what do you recommend that I read," which we have firmly come down against.
This is the case whether the question is "What volumes of this series are essential and which can I skip"; or "What ordering of this author's works will best make me appreciate his themes and ideas"; or "I know the publication and chronology orders, but maybe you have a different reading order that makes reading the series really fantastic." These are all great things to discuss, but they are very poor Stack Exchange questions, and will have all the problems we're trying to avoid with reading recommendations.
Effectively, my position is that requesting lists, orderings, subgroups of books is fine; requesting recommendations for reading order is just another recommendation quetion.