Is there / could there be a tool to restore the iPhone without itunes? (from an original firmware / system file) or ...

It could allow us to backup the iPhone to a "disk image" and restore it at any time to that state. (like restoring from dmg with diskutils in OSX)

Imagine not having to restore after a broken mod or some stupid error! Then there are all those itunes restore errors... and, even after a successful restore, you have to do everything from scratch.

And one more use: if a future firmware update from Apple renders previous mods useless (or even the Turbosim workaround) doing a restore will automatically update to the latest firmware.

so, what do you think?

This question has been asked and answered in various threads previously.
Tools exist to do the backup (mostly used by forensic types). There is really zero use in extending these tools to "restore" as iTunes already includes a much more efficent system which restores only the actual user modified data (in addition to re-syncing static data like apps, video and music).

Disk imaging for restore is a topic best left to the people who make the OS, they are the experts on what should be excluded from the backup to trigger the appropriate "post restore - first run config" and to ensure that any "device unique data" is not inadvertently restored to an (apparently) identical device.

I don't think you entirely understand the complexities involved in this topic as is evidenced by your last comment. For example, the use of a disk image backup to revert an accidental iOS upgrade will not downgrade baseband or other integrated device firmware that may be updated by Apple's update routines (battery firmware for example)

Regardless, thread closed - this is a duplicate and has now been answered.