Okay here you go, plan 1
There's a loop for running a passenger train and freights. The roundhouse is figured prominantly as a place to showcase locomotives. The turntable is the good ole Atlas 9", but you could fit a larger one in. HOWEVER, the minimum curve radius here is 18" so larger locos wouldn't look at home anyway. They may make it around, but may not look all that great.
The station labled 'E' is the main passenger depot. You could park a nice passenger train on the main just to the left of the depot. I'm thinking that nice 4 car set from Walthers/Rivarossi of 60' cars along with an USRA light Pacific or an F unit.
Boxes A&B are industries. You decide, but you've got room to make them larger, maybe the Walthers cornerstone series. Possibly a cement facility...I think they just came out with something like that. This would tie in well with an idea for the branch line...
Take the siding to the left of the roundhouse and you head up the branch line. I'd use the Woodland Scenics risers to get a steady, dependable grade, maybe a 2%, I didn't do the math to see what you'd need to cross over the main. Anything more and you'd want to use geared power, perhaps. Slower engines can make a layout seem larger.
At the end of the branch is a little town. 'D' is a passenger depot and 'E' might work well as an aggregate industry, maybe limestone or some kind of quarry. This could tie into the industry in the town down below. Make it pulpwood and put a paper mill in the town. Make it coal and put a power plant down below, you get the idea. This way cars can move from one place to another on the layout in a dedicated way.
That leads to a potential drawback. There really is no staging. You could hide a train in the tunnel, like the passenger train or a freight, while running a local up the branch. Another drawback, I'm not sure how well the turnouts would work in the yard, how closely spaced they could be. I tried to not use any curved turnouts.
Scenically, I put a creek or small stream running between the climbing branch line and the yard. This serves to add a little distance there and put a break between the busier trackwork in front and the simpler, more scenic trackwork in the back. I'd have a major mountain or significant outcrop at letter 'F'. This serves to hide the main line as it disappears behind the branch. Whenever trains disappear for even a few seconds it lengthens the run and in our heads makes it seem like they've gone somewhere other than just around a loop.
There's plenty of room for more structures in the town below around the roundhouse, like engine servicing facilities. Also a few other spurs in the town at the end of the branch. Operationally, through freights set out/pick up blocks of cars destined for the local industries and destinations up the branch. A nice little yard goat, like the USRA 0-6-0 or BLI's latest SW something or other with sound could work the yard, blocking cars for the freights and shifting the local industries.
Okay, there you go. If I get any more ideas I'll sketch them out, or if you get inspired, let me know. Good luck with the demo/benchwork.
Galen