Collecting Blish Series

OK. This is the way to do it (a "Why didn't I think of this before?" moment):

I. The Haertel Scholium Prelude: the early interconnected stories and the Galactic Cluster Trilogy.
II. The Heart Stars: the Jack Loftus novels.
III. After Such Knowledge.
IV. The Seedling Stars.
V. Cities in Flight.
VI. The Quincunx Tetralogy.
VII. The Haertel Scholium Coda.

Thus:

"Haertel Scholium" volumes would open and close the sequence;
the "Coda" would be read at the end where it belongs instead of in the opening volume;
"The Haertel Scholium" and "The Heart Stars" form a fortuitous verbal sequence;
the latter is an appropriate title for the diptych, The Star Dwellers and Mission To The Heart Stars;
The Heart Stars would also be a companion title to The Seedling Stars. 

Okie Ys?

Poul and Karen Anderson wrote a historical fantasy tetralogy, The King Of Ys, about the fabulous city of Ys which was inundated. James Blish wrote a futuristic science fiction tetralogy, Cities In Flight, about New York and other cities that become "Okies" by flying through the galaxy with anti-gravity drives called "spindizzies." (Ys and New York have in common that they are Atlantic ports with towers.)

How is this for a crossover idea?

Alternative timeline: Ys survived and will become an Okie.

Implications:

(i) By leaving the Atlantic, Ys would end its Pact with the sea God, Lir - unless its spindizzy field was vast enough to enclose a sizeable volume of the Atlantic (which would cause problems when the field had to be switched off)?

(ii) This version of Ys exists only in a fantasy scenario. Thus, on each new planet, the Ysans would have to deal not only with that planet's inhabitants but also with its Gods.

(iii) Each new King of Ys must be both a foreigner and able to marry the Nine Witch-Queens so non-humans need not apply. Possible candidates would include colonials or maybe an Okie who "wanted off" his current city. He would challenge and fight the current King with futuristic weapons like the Bethe blasters mentioned by Blish.

(iv) It seems appropriate that, if there is a multiverse, then Ys should be able to exist in a fabulous future as well as in a legendary past. Gratillonius, the "last King of Ys", and a later Okie King of Ys would be able to meet in the inter-universal inn, the Old Phoenix.