I think the main problems of Unix today is that it's not Unix-style enough. Too many namespaces with too many non-composable separate APIs on them instead of "everything is a file". Plan9 is more Unix than Unix and that's indeed better. Redox OS, too.

The Unix security model is mostly useless today, but it seems like something better is possible as an incremental change, and there are projects that do that, like RSBAC.