Personally I would like additive and removable layouts. While being able to choose where the layout is attached based on the top/left corner.
I would like the option to have the layout load int he background and add when ready. When I remove a layout then the assets of that layout is removed providing another layout is not using them.
I would ultimatly like to make an expansive GTA2 like world with a large distinct sets of graphics for area. However the big issue I had were
A. Memory overload for all the different regions(Downtown, suburbs, forest, ghetto, industrial, water, park, subway....). there is just no way to handle this especially when you then also need to add sounds that are also specific to regions. forest sounds are not needed in the subway... etc. one big layout make this not really feasible.
B. I found that the scrolling in the editor seems wierd at the very large size such as 100,000 x 100,000.
Also the stitching with layout additive and removing can work with say layering. Here is an example.
Player is in car.. vroom.
Player get's out of car.
Player walks into build.... lobby is loaded. while in lobby load rest of floor, basement, floor 1. on floor 1, unload city, region A, load floor 2.
player walks out of build. unload floor 1, and basement.
player drives to subway
player drives into subway
player is in a tunnel undergound. and can traverse the city.
by using additive and removable layouts we can get streaming load/unloading of city components. I have no idea how this would impact the current engine or the requirements. however as seen by the sample 1 layout would not work for urban environments that want to allow a truly seemless world.