Hi there,
I think a great plugin would be for a "search refine" feature. This is becoming the norm in property listing sites (as well as many other sites). It uses Ajax to narrow down the listings with the use of Sliders, dates form etc... With even Google relase of Google Instant, the expectation of modern sites will be to be able to show results from within the same page. See sites such as homeaway.com thefind.com
Feel free to contact me for more details... Thomas