I realize you have good intentions, but do you really think creating another competitor to RPM and DEB is a good idea? I think it's just going to increase fragmentation.
It seems like the feature you want (atomic updates) could be retrofitted to the existing systems. btrfs supports the concept of transactions at the filesystem level, which is exactly what you need here.
I'm pretty sure that the RPM guys would be overjoyed to have someone sane take over as the upstream maintainer. Please, please, please, think before you reinvent the wheel.