Carvac means that XMP can contain a ton of (arbitrary) data. What do you expect to work from a XMP file? Just tags? Date / import stuff?
I know of people who expect the edits from Lightroom to show up perfectly in another program because they are in a XMP.
Carvac: think about why you code. Do you get joy out of other people using your software? Then listen to ideas they have :). Do you use it for your own usage or workflow, do what makes you happy
you need to find a balance in this. I’m not going to say which way to go, but I find it helps a lot to know what the reason is to do it :).
Now…What I like about 'a database is that it k ows stuff from my pictures, even if they are not on the system.
Having one collection in online backup with a local copy on a NAS means that I don’t have all my files on all my laptops. I use multiple machines to ingest,multiple to edit.Depends in where I am at the moment :).
Having a database of everything, means I can search the database (with small thumbnails) to find files that are ‘in storage’, not on the system.
Having stuff (only) in sidecars makes it more difficult to search, since xmps need to be found and read. And if not everything is on my system,I can’t search sidecars that are online in storage.
So for speedier searches,programs often need some sort of database. Writing sidecars then feels somehow redundant. They always feel like clutter to me.
They help with transferring info from program to program… Yes. But that’s something I seldom do,so I don’t want them around.
Being able to write them helps to not be ‘locked in’ to a single program,so that is a big plus for sidecars.
Acdsee has a thing where they write the xmp section back into the files, so they are always with the file (and copying the file copies the metadata you added like tags). Sounds nice, but changing the source files seems wrong to me (the fact that they are hashed on my sotrage means they are not meant to change a single bit :)).
About ‘film rolls’ (or collections, or imports)…
I learned from Acdsee that hierarchical tags are good for everything. It doesn’t have a concept or albums,groups or collections. Just the folders how I ordered them.
But add a tag 'film rolls, and add a tag under that. Assign it to images (during import for example) and there you have it. Nothing is stopping you from adding images to more than one filmroll this way. And also you can still add tags for things like people, locations, etc…
So…Maybe keep the database and build it how you see fit. But add an option somewhere to export xmp files next to source files. As a single action to do when someone wants to transfer info to other software.
And maybe import xmp the same way (with an option to delete the xmp after import 
).
As for functionality, I think hierarchical tags and ratings are the basics.An exif block (or the same fields as basic exif copies) is also expected I think (gps, make,model,dates, title, author. . To name a few.
And I think people will expect it to use it as a backup or transfer to other computer kind of thing, so serializing the filmulator settings to it is probably also what people expect.
Personally, I’m glad my folder organizing is still enough for me… But the time will come I need more :s.
