Wow - can I just say - have you bitten off a huge task or what? I love the concept of this but see it as being monumental in getting the datasets to be able to build the database you require. Mind you in saying that, if you were specifically looking at constructing it to do just the New Orleans area then maybe it is achievable without an enormous amount of data mining. You do say though at the beginning of your post that you are looking at software to look at data for any city. I really admire your efforts and I hope you pull it off. I for one love the idea and have already found by pawing all over 18th and 19th century maps of Greater London I am getting a better idea of where my own ancestors lived and worked. I find it fascinating and I am sure if you are able to put together software that will make this an easier task there would be people out there wanting to purchase it. The hard bit may be in distributing the information to User Groups (eg Family History Societies etc) in a way that they easily comprehend what your software is capable of and what it makes a lot easier.
Best of luck - I really hope you pull this together or maybe already have.
..Phil - Perth Western Australia