After some consideration, I've reached the conclusion that there are, at present, three loose 'types' of Stronghold; Interior, Developed and Natural.

INTERIOR:

Interior Strongholds are those strongholds with minimum outdoors elements, usually just a balcony or landing pad. These include the Fleet Penthouses, Capital World Apartments, and Nar Shadaa Skyhook.

DEVELOPED:

Developed Strongholds are those where the exterior of the Stronghold is as important as the interior, usually a medium amount of buildings or structures balanced by an equal amount of open (or relatively open) space.

I classify Tatooine, Rishi and Alderaan in this type.

NATURAL:

Natural strongholds are those where the majority of the stronghold is open outdoors, with only one or two buildings. Yavin 4, with it's one main building and two one-room towers, but good-sized front area and absolutely massive 'courtyard' is the best example of this type.

There are a couple of strongholds, Manaan and Umbara, that straddle the line between Interior and Developed, but for the most part I think those three types contain all the different strongholds. Possibly, the typing I've used could be used to determine what kind of 'new' stronghold might be needed.

(That's right, Dantooine Farm fans; a 'Natural' stronghold would be perfect for that...)