This is something I've been trying to implement in my own yard, as well as marketing them in our little home based nursery. Here's some of the things I've had good results with....
I bought some of the taro bulbs from the supermarket and planted them in the "elephant ear" bed, and they are just as pretty as the ornamental cultivars.
Cotton & okra plants are good substitutes for ornamental hibiscus.
Peppers, like tobasco and poinsettia, look nice in the autumn landscape when left unharvested. The various shades of green, yellow, orange and red peppers on the plants catch the eye. Sweet lemon & tangerine peppers aren't as colorful, but still look nice in the flower beds.
Dwarf curled kale and the multicolored chard are nice in fall/winter.
Bunching onions are a good filler, especially when in bloom.
The ornamental purslane has pretty flowers and tastes the same as the "wild" type.
Runner beans aren't as aggressive as trumpet vine or wisteria, yet they provide plenty of color.
Looking forward to seeing what others say!