Best advice I can offer you is to search with your new best friends (yahoo, google, ask, answers, bing) as this has been covered before in many a forum. In fact, searching yahoo with vb6 thesis ideas returned 19400 hits...
Before deciding which proposal i think you should choose between popular business application or others.
For popular business application the easiest way is retail application, but you have to find the company to collect the data because it has many different type like supermarket, chemist and other it has specific requirement. Others could be manufacturing, travel agency and many others. i try to give some examples
Beside popular application you can have library, multimedia or even web browser
You can try to search the samples from other universities or college over the net