One item that many new model railroaders sometimes forget is to
make sure that they can reach all areas of the layouts because work
will always need to be done. You can't just fill a room with the
layout. There are several solutions to reachability. The
best one is to make sure that every part of the layout can be reached
standing on the floor at the edge. Other solutions include
pop-ups in the layout (probably best reserved for non-track, minimally
scenicked areas) and "duck unders" with or without removable sections.
Don't forget that you will also need access to the underside of the
layout for electrical work.