One of the only things mentioned for sure in the canon materials about the Harmonium (on the Prime) is that its vast. Its also (generally) expansionist and unlikely to settle for 'just' one entire world and eight colony worlds, as detailed in the Ortho project PDF.
In my opinion there are likely to be many places where the Harmonium has a toehold on a world but is far from dominating it politically, everywhere from a few outposts and bands of missionaries to whole kingdoms and alliances of nations that have joined the cause and are fighting to unify their world as Romhel did. These would be the real frontiers of the Orthorian Central Authority, both civilised and barbaric, with plenty opportunities for Harmonium adventures, from taming teh wilderness to diplomacy to crusades.
To quote Chiwetel Ejiofor: "we're building a better world. All of them. Better worlds."
I think that it wouldn't go amiss to have a paragraph or two on some sample worlds the Harmonium is interested in but certainly hasn't conquered, and that includes the old published settings and any homebrewed worlds that their authors would like to contribute.
So, where would (if any?) the Harmonium have outposts on:
Athas?
Eberron
Faerun (I could see them having contact with the Land of Fate/Dragon Emperors of Kara-Tur rather than the usual areas)
Krynn
Oerth
Anywhere else we can think of or make up....
Anybody feel like throwing some words in here please do so!
(I will, as soon as I have a chance)
I think we should remember that this isn't a Harmonium supplement and the 'be-end-all' of the Union. This seems like we're going more than slightly beyond what we have.
Also, while the Harmonium is vast. I think we should also keep some limits to it.
This is stuff for the Planar Harmonium to handle. It's still an Ortho based group, not the Galactic Empire.