As usual, I caught a bug on the trip to Michigan last weekend. Air line flights always get me. So while I am home sick, I started thinking about how could I expand this layout. This speculation was based on three main factors.
First is that I plan to start laying the track at Aquia Landing soon. That is the last section of track work in the current plan - the "light at the end of the tunnel." I saved it for last as I knew if I built the harbor part of the layout first, I would probably never have finished the the rest of it. Essentially I saved the best for last. But, I always like to have a a carrot dangling in front of me. It is too depressing to think this is the end. To me layout planning and building is the best part of the hobby, model building, scenery, and operations next. Track laying, DCC and wiring are last. So I was thinking about expansion.
Secondly, my wife is convinced that we are not going to move when we are retired, even though I am lobbying to move to a lower cost area. I never win these debates. So it looks like its, "Die in place," as we used to say in the Army. That means this will be my last basement I will have to work in.
Third, several of my model railroad friends have had to abandon their railroads as age related problems set in. Sad but inevitable. Might as well plan for that too. So how would I expand this to my lifetime layout?

Earlier, I drew potential expansion plans showing Alexandria, and Burnside Wharf as possible expansions. The newest plans look at the other end of the railroad. They address the most troublesome aspects about the current layout, notably the tight curves at the north end of Falmouth, including the turntable lead and to a lesser extent at Stoneman's Station.
The first phase would be to build a new Falmouth on a narrow shelf where the road show layout is now. It would reside on top of a long set of custom bookshelves with room for a TV underneath. Once the new Falmouth was built, the current Falmouth would be removed, and the tracks reconfigured to pass through the closet. Stoneman's Station would move to the current Falmouth location. The turnouts at current Stoneman's would be removed and it becomes a nice long stretch of single track. All the new curves are 30 inch minimum radius with easements.
The track section in the closet would be removable to allow access to storage when the layout is not running. There is room for it for fit. I used that closet for staging in an earlier N Scale layout.

The next phase is a bit more drastic as it repurposes the TV room to purely layout space. Not a big loss as I rarely watch TV. In this plan I would add a central blob adding about 27 feet of mainline run between Falmouth and Stoneman's. The aisles would be fairly generous, but not enough room for a TV watching area.
The last major change not shown on these drawings involves rebuilding the Potomac Creek bridge to its full size, about 8 feet long. Yes, once the layout is expanded there is no compelling reason not to rebuild it. This would only be possible if I move Stoneman's Station. If Stoneman's stays where it is, then a bigger bridge would not work.
The obvious advantage of these plans is that there are longer runs between stations, an important factor in TT&TO operation. I plan another post soon about TT&TO as it will apply to this RR. This is an subject that I have been examining lately as I seriously plan for operations.
The track laying and wharf building in Aquia Landing should take about a year. Throw in another 6 months for freight car construction then operations can start. The ships and other structures will take perhaps another year. That still leaves me about 2 years until retirement. Plenty of time to think about this. Once this layout is maximally expanded, I do not plan another basement filling layout. Instead I would continue to build smaller projects until no longer able or take up another hobby.
In answer to Marty's comment, I added am image of the interior of the closet showing the split landing steps. I keep my shop vac, photography lights, tripods and a lot of train stuff in here. The landing step is 52 inches off the floor, just enough room to have a track at 48 inches below. Installing it will be a pain as there is another joist that will have to be cut on a angle, but it will fit.