Okay here's what I've settled on so far. Don't know if its most elegant or how I could improve on it. Please let me know.
Ex.
Wizard's First Rule (Part of Darken Rahl saga which is 1st)
Phantom (Part of Chainfire saga which comes 2nd).
Note that the saga order naturally doesn't follow alphabetical order.
So I've decided to use hierarchies in series column to classify them respectively:
Sword of Truth.Darken Rahl [1] (Wizard's First Rule)
Sword of Truth.Chainfire [2] (Phantom)
This tells me that
Wizard's is the first book in Darken Rahl Saga of Sword of Truth universe and
Phantom is the second book in the Chainfire Saga.
Ideally I would just use a subseries custom column but I don't want to add yet another column to the book list view to see at a glance which universe/saga a book is a part of.
Instead I created a hidden column seriessort (I don't know which column type I should use, I went with long text. Should I do otherwise?). Then for
Wizard's First Rule I set it to 001.001.
Phantom would be 002.002.
Now I can sort by Author, then Seriessort. This way Darken Rahl saga will show up first instead of Chainfire.