If you allow dependency-included self-host (preferably via an application wrapper like Electron where I can use a firewall to control which outbound connections are allowed), support mobile devices, and charge a one-time fee for p2p sync (decentralized, like SyncThing), this will be an instant buy for me.

Thought: Can you add a logout button?

Also, please consider a name change so SquEnix doesn't sue you out of existence!