Admittedly, though, replacing a full OS isn't likely to happen as users reject anything without feature X, app Y, or benchmark Z. The thing is there's a lot of those things in an OS to the point that barrier of entry to building one is insanely high.
Best shortcut to that I've seen are the Rump kernels.
Well, not really. Rump kernels are a godsend shortcut (similar to DDE, but much more advanced) for device drivers, but Blub-loving users wanting their apps means you still need to roll a POSIX userland of some sort (the way GNU Hurd does it over Mach is quite interesting, it supports things like processes with multiple uids with all POSIX calls being RPC that delegates to object servers) or since we're talking about a mainstream improvement here, Darwin/XNU and maybe even Windows NT. No escape.
The L4 kernels have just been porting Linux, etc to user-mode to run the untrusted apps with calls to underlying API. Might be the best that can be done outside some kind of source to source translation with heuristics.
Best shortcut to that I've seen are the Rump kernels.
Well, not really. Rump kernels are a godsend shortcut (similar to DDE, but much more advanced) for device drivers, but Blub-loving users wanting their apps means you still need to roll a POSIX userland of some sort (the way GNU Hurd does it over Mach is quite interesting, it supports things like processes with multiple uids with all POSIX calls being RPC that delegates to object servers) or since we're talking about a mainstream improvement here, Darwin/XNU and maybe even Windows NT. No escape.