Borrowing from my past life : Using similar sized tables I added locking casters to the tables so I could open a corridor down the center for play or setup then butt them back together again, tables still folded and access was very good. In Warhammer we used a series of 4' modular layout boards which allowed us to re-configure our terrain any number of ways which included mountains, shoreline, flat and forested pieces. I look forward to seeing your ideas.
Gepetto
A modular tile system could work well and may be necessary to allow different layouts to be be switched in and out quickly and easily. I'll start with the plain tables and a cloth on top, but tiles might also add some stability to the tables with some extra weight. I could hotglue trees, rocks, castles etc to the tiles, and store spare tiles in the cupboards. 3' x 3' wpuld be a good size for a big tile, with 1.5' x 1.5' tiles as a chanegup, so scenery can be easily adjusted.
Locking casters is an interesting idea. I haven't fixed on the tables yet, but as they'll get years of use I'm not just going for the cheapest option first.
Under my
playroom den is a double garage which will be developed as a woodshop, primarily for my wife's use, but I'm sure she would help me cut plywood tiles to the required size.
As Richard notes, once you have more space to stretch out, the possibilities multiply. We didn't choose to move to the USA just for extra space and a bigger house, but we certainly chose the house we are buying for the possibilities it offers for the two of us to have more dedicated hobby space.