Bill said in the interview he did with GQ a couple months ago that he would do another Ghostbuster's film if he liked the script, the key word being HE. So if they present Bill with a script and it blows him away then look for actual work to start and casting news to flow. I don't expect we will hear anything significant any time soon though.
I like Sammy's idea about one of the Ghostbusters getting hurt and yeah where is Winston in that though? Did you guy see the pic on that Ghostbusters III blog of Eliza Dushku wearing a proton pack?