The question of what should be in the secondary hull comes to mind. An important question to ask is--What should the proportion of working space, cargo space, recreational space, and crew quarters be?
Assuming that ships like the Saladin, with a crew of around 200 did exist, then would work/cargo areas found in the saucer be filled with crew quarters instead, to accommodate the larger crew on a Constitution? Or would the extra crew quarters be found in the secondary hull along with all the extra recreational facilities that apparently exist? The recreation area seen in TMP would likely have to be omitted on a refit-Saladin, if there were such a thing.
In reality it would probably be a mix of the two. But the more I think about, the more that it seems that this would make each saucer much more of a custom job, which may or may not be desirable. On the other hand, it also seems odd that an engineering hull, supposed to give more power and a larger deflector, would also be full of recreation and crew facilities.
In any case, all this seems to be a good argument for there being at least two engine rooms that look the one in TOS: one for the saucer that would be the only one on ships like the Saladin, Ptolemy and others, and at least one in the secondary hull for whatever increased power is down there.
I think it is an idea worth considering that the secondary hull has two engine rooms: one with the pipe structure facing rear to the nacelles, and the other facing forward with the pipe structure going to the deflector, with the raised Emergency Manual monitor between them. In TOS we see characters working there and see one engine room behind them, but there would be another one in front of them, where the camera usually is. I'm not sure that she show ever showed the wall opposite the ladder in TOS, though at least one game shows it and prevents this notion, albeit only with a heavily modified ship.