Ken,
We have a space very similar to yours and are just over 1 year into the construction now. The track plan was the toughest part for us, and we toyed different tracks plans using RR Tracks for over a year before we finalized on,e and got serious about building. I personally, can't design or plan things but I can build it. So I struggled with the track plan, even knowing what we wanted to do with the layout as far as running trains.
I'm glad you don't want any yards or a RH because it's really hard to fit that stuff in. We got in the TT and RH, but they are the main focal points of our layout, with trains running on 3 different tracks at different elevations. The yard was on the wish list, but sacrafices had to be made to get everything else we wanted to do.
Something like a folded dog bone track plan with give you a double loop so to speak, and crossing over itself, will create the change in elevation. Running around the outside edges should allow you to easily incorporate all your bridges, just remember to build your bench work lower in those areas to incorporate the scenery. Having long passing sidings is a big help, as they can be used to park full length trains, and a waiting point for another train to pass in the opposite direction. I would highly suggest reverse loops as well, so you can reverse direction...in either directions. You don't want to be stuck with a train going the same direction all the time.
I can't help design a track plan, but I can answer questions as to what we have learned, or what we did, and why. There is a thread in this section of our layout, and clicking the YouTube link in my signature at the bottom of this post will show you our progress if you want to check it out.
One final thought that just about everyone here will agree with, if you want 072, stick with that...always go bigger if you can. Don't compromise and put in a smaller curve or tighter switch, re-design the track work to fit the bigger ones in. We have a couple 064 switches in the diesel service area....everything else is minimum 072, with 080 and 096. We used Ross switches and all the cross overs are Ross Regulars (11 degrees) for the mainlines. Your space is big enough to run articulated engines if you want to, so stick to your guns with 072 as a minimum and build everything to accomadate the big steam.
