We are going the long way around to figure out that Tannenbaum was right and we need microkernels. The IPC costs cause sticker shock, but at the end of the day perhaps they are table stakes for a provably correct system.
At a low level and at a place of ignorance, I kind of wish monolithic kernels hadn't won, just like I wish the morbidly obese browsers hadn't either... They sound way more, well, sound in their theoretical basis
I think one of the biggest ones I recall, and I think it was part of the L4 secret sauce, was the idea of throwing a number of small services into a single address space, so that a context switch requires resetting the read and write permissions, but not purging the lookup table, making context switches a couple times faster. With 64 bit addressing that gets easier, and allows other tricks. For instance recent concurrent garbage collectors in Java alias the same pages to 4(?) different addresses with different read/write permissions. Unfortunately this means your Java process can 'only' address 4 exabytes of memory instead of 16, but we will surely persevere, somehow. How that plays with Meltdown and Specter might put the kibosh on this though, and most processors don't actually support 64 bit addressing. Some only do 44, 48.
That's pretty old news though. The more interesting one to me is whether there's a flavor of io_uring that could serve as the basis for a new microkernel, allowing for 'chattier' interprocess relationships to be workable by amortizing the call overhead across multiple interactions.