I would suggest you have a look at
https://www.lastdodo.nl/nl to see what basic information would be required. It, also, will show you the problems with creating an app for collections of different types.
For an individual stamp collector, information on the actual stamp in the collection, also, would be required.
Certificate numbers and issuers, as suggested, would be very interesting.
Not just price paid, but also the source may be interesting (keep in mind a platform may also involve a seller).
You might want to add specialist items, or formats (e.g. block of four, or cylinder block, sheet header, etc.).
Colours, types, dies, luminescence (incl. layout, colour, etc.) etc.
For stamps with overprints: similar information might be required for the basic stamp and the overprint, with a cross reference to the original stamp.