Rich, I'm still looking at the mountain/tunnel solution as a strong contender. It's sort of a logical sequence: to move at a reasonable speed requires some straights, the curves demand enough room for their radii, and that leaves you with a lot of perimeter track with a big void in the center you can't use for the train. Elevating to get more than a basic oval means mountains/bridges. Once you go that step, you have to 'justify' the layout, i.e., have it make sense, visually. By that, I mean an elevated loop that goes nowhere and does nothing sort of offends the eye.
I'm about 6 months away from starting; right now, though, I'm leaning toward a central lake with mountains in the back (with tunnel line plus elevated line on ridges over it). Foreground would be at ground level and climb/descend on the sides. Finally, I'd like to have turnouts that allow my trains to leave the table to travel to another table and return. This all assumes my plan to find a place to rent with a perfect basement works out. Best of luck on your layout,