Let's discuss the creation of a personal tool to manage your coin collection... What does it need to have, and how to go about creating it?

My personal take on it (in bullet points):
* it needs to run on all sorts of operating systems; both regular computers - desktop/laptops running Windows, Linux or Mac, and mobile devices
* have a local database with enough information in it to make it useful even if the device is not connected to the internet (i.e. be able to function independently from any external source)
* but also, connect to a site such as Online Coin Club so that you can a) enrich the local information with data that is downloaded from the site, and b) back up your data to the site so it can be moved from one of your devices to another

In terms of features, I would like to have the following functionality:
* ability to track what I have (and not just a checkbox "I have it" - it needs to handle having multiples of each coin, and also what grade they are in)
* wish list of what else I want to have
* track where I got the coins from and how much I paid
* track sales/swaps
* ideally, also get some news from external sources so I can follow what new releases there are

I know some people are already working on this, so can I get some opinions? What would you put in such an app? How would you like to connect it to Online Coin Club (what APIs would you need)?