FUSE is what a microkernel filesystem would look like. There are some optimisations that FUSE doesn't do, that microkernels usually have. In the most extreme form, L4 trims down communication primitives to the most efficient platform-specific ways of exchanging memory buffers. In all cases, microkernels and FUSE still need context switches for everything, and those are expensive. If you leave out the context switches, you don't have a microkernel anymore. This is what Windows did by pulling graphics drivers into the kernel, because context switches are slow.
So no. Microkernels have been tried. Microkernel-workalike filesystems are here with FUSE. They suck because microkernels suck when you need performance. Research has gone into different directions, like microkernels as hypervisors and for security, because it has become clear that the performance problems if microkernels are inherent and unfixable.