RW,
Your really asking about "kei-do" the Japanese art of display. It's pretty complex and extremely subjective. It can take years to learn the intricacies.
I've not studied kei-do, but I have been buying scrolls to use in bonsai display for years. The most important thing I've learned (just like bonsai) "less is more." Simple is better. The more complex, the more confused your "message' becomes.
A single element in the picture, or two at the most, is perfect. Don't try to "say" too much with the scroll. It's meant to suggest, not be explicit. It should be subtle enough to allow the viewer's imagination some room to run.
Good "bulletproof" choices for scrolls include: Moon/clouds, single insect (this could include fireflies, spiders, grasshoppers, cicada, etc.--all of which are pretty common in smaller kakemono) on a leaf/grass, autumn flowers with moon, waterfall (alone, no surrounding foliage), single or a few birds.
Here are few examples:
http://cgi.ebay.com/JAPANESE-SCROLL...ewItemQQptZAsian_Antiques?hash=item3a546a3f58
http://cgi.ebay.com/3514-Vintage-Ja...ewItemQQptZAsian_Antiques?hash=item5ad492cd55
http://cgi.ebay.com/3457-Japanese-H...ewItemQQptZAsian_Antiques?hash=item4a9bcdaf87
http://cgi.ebay.com/1901Z-Japanese-...ewItemQQptZAsian_Antiques?hash=item2ea8c606a7
Scroll selection is also highly seasonal--some scrolls, like fireflies, cicadas, certain flower, suggest summer. Bats can suggest late summer or autumn. Depending on background/in picture treatment, moons scrolls can be almost any season..
This is just the tip of a very large display iceberg. Search on Kei-do to find more. Search ebay (japanese scrolls--sort highest to lowest) for less expensive (and sometimes pretty nice) scrolls.
Also, if you're going to display your bonsai and scroll in a show--don't try to make a scroll yourself. Stay away from Kanji and calligraphy. Any translation is usually wrong, sometimes laughably so. You don't want a Japanese visitor to pee himself laughing when he sees your display.
I've purchased a dozen or so scrolls of all sizes from Ebay vendors based in Japan. I've never paid over $100 for one. Shipping from Japan adds only about $5 or so. The vast majority of the other sources I've seen online are pretty expensive and/or the quality isn't there. Some are outright rip-offs.