You can do the automatic sign ups with extensions pretty easily.
As for the offline access native apps: does each site then have to write a native app module for your browser for it to work. This doesn't sound like something you can do without work from each site owner.
The same for locally stored data. You could keep a local copy of any files you upload to sites, but if you are uploading, you already should have a copy. But you can't do anything in a browser to make facebook serve images from your computer instead of theirs. This is another feature which requires work from every site to make happen.
As for the offline access native apps: does each site then have to write a native app module for your browser for it to work. This doesn't sound like something you can do without work from each site owner.
The same for locally stored data. You could keep a local copy of any files you upload to sites, but if you are uploading, you already should have a copy. But you can't do anything in a browser to make facebook serve images from your computer instead of theirs. This is another feature which requires work from every site to make happen.