• Near instantaneous snapshots and rollback (would help with system restore etc)
• Compression that uses a modern algorithm
• Checking for silent corruption, so users know if their files are no longer correct

I’d add better built in multi-device support and recovery (think RAID and drive pooling) but that might be beyond the “average” user (which is always a vague term and I feel there are many types of users within that average). E.g. users that mod their games can benefit from snapshots and/or reflink copies allowing to make backups of their game dirs without taking up any additional space beyond the changes that the mods add.