I think a link would be preferable to a file for logging in to accounts. A link could be stored in text-only places such as Bitwarden, sent through email, and allows the user to log in with a single click instead of going to wallet.nimiq.com and uploading a file. A link can also be sent from desktop to mobile via QR code.
Maybe GUNDB could be used for storage and syncing of labels, contacts, private keys, and other information.
I think it’s a cool idea! 
It would basically work like cashlinks … but it would be loginlinks. Right now, you can scan the QR code on a login file with your mobile to login there, so that part works already. But sending a link somewhere indeed might be easy. E.g. you forward it to your phone and open it there, enter your password and you’re done.
I think though, there are use cases for the login file as well. Both are interesting IMO. It’s nice to store a file somewhere that allows you to login - for a link, you need a document or so to store it. The graphic file has something tangible. The link could be better for pro users.
About the syncing service, yes, there is nice technology out there to be used. I didn’t know about GunDB but yes, that’s cool! I could also imagine to use IPFS (I’m more familiar with that) - in any case, I think this is technically not too hard to make, but it requires time and effort of course. It’s on the roadmap for sure. I’m wondering, if that could be a community effort… but it has severe security implications of course!
Right now, scanning it with the normal camera searches the web for a long string of random letters. Maybe there is a way to do it inside of Nimiq that I couldn’t find.
I do think there are usecases for a login file, and I don’t suggest replacing it, just adding to it.
Regarding IPFS, I have looked into both and it seems that GUN is lighter, web-native and better for storing small amounts of data when compared to IPFS.
Also of note:
Torus logins would be much easier to use than either a login file or login link.