@(eval 'Toast) Microkernel designs give you some tools, but a microkernel that includes a large set of servers, all developed together, all with tightly bound interdependencies, is not much more approachable than a monolithic kernel with loadable modules.
I’m more inclined to look from the other direction. Look at plugin and expansion APIs in proprietary software and ask ‘what can these not do because the vendor doesn’t want to expose their implementation or enable competition?’ And then try to build things that enable all of those things.