Surely there must be more than just these three uses for an OS, right?

Is there any justification for a microkernel to exist in an OS? The modular approach isn't much different from this.

The layered design of an OS may be described as follows: I'd like to know your thoughts on this tactic, because I'm interested in exploring it more. Rather than relying just on words to make your point, why not illustrate it?

Think about the obstacles you'll run against as you construct the framework, and investigate any aspects that might scuttle or derail any given choice. Does cloud computing provide a solution to these issues? Is there a risk that anything bad may happen?

Is there anything that could go wrong with an OS if it had an architecture that allowed for a lot of threads to run at once?

The original Linux kernel wasn't preemptible because of a decision made by the programmers who made it.In a single-processor system, a non-preemptible kernel serves no use. What I don't get is why a multi-processor machine would negate the gain.