Eros and KeyKOS had some very interesting ideas in addition to capabilities.

Global process persistence meant that you could pull the plug on a running system and start it back up right where it left off. It was constantly journalling system activity onto permanent storage. This also provided some level of system roll-back, kind of like a database but applied to the whole OS and all its activities.

Something like that could be very useful on low power intermittent use devices like smartphones.