Zachary
Your minimum radius is going to be the key issue as to what type of design will work for 5x9. If you stick with '50s era with no or "shorty" passenger cars and small steam and/or early diesel, 18" radius curves will work pretty well. There are several designs for both point-to-point and loop layouts specifically for 5x9 in HO.
If you want to run larger equipment, you need bigger radius curves. In that case, taking a 4x8 plan and redrawing it with 22" radius curves would be about the best you can do. Here's one of my favorite 4x8 plans which has both continuous run and point-to-point branch line action. It's called Plywood Summit Lines (Atlas plan #36).
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As I said, you would need to redraw this using software such as Atlas RTS (free download). Or, build your table and lay it out full size with actual track on the table. Then cookie cutter the raised sections of the table. The other option is to live with the 18" radius curves and add 9" worth of straight on each side to various tracks. The last has the advantage of moving track away from the table edges.
If you want more than 22" radius curves, you can go up to about 26-28" radius curves by building a simple oval with some industries and spurs inside the oval. See Harold Minkwitz's great track plan (
http://www.pacificcoastairlinerr.com/4x8/trackplanning/) for a 4x8 that could be redone with the larger curves. Note that you would need to use Walters curved turnouts for one end of each of the passing tracks to get the maximum curve radius.
Hope this helps