One of my "dream projects" (maybe after retirement 🙈😁) would be to create yet another :commodore: #C64 OS. One that works on a vanilla unextended machine.
There are quite a few around of course. To make something meaningful, you have to think about what could give #retrocomputing enthusiasts an excuse to use their breadbin once in a while 😏
I think it should just support very basic #Internet service clients, like Email, IRC, BBS (via TCP), ... of course this means to require one hardware extension (apart from your obligatory floppy drive): Some #ethernet (or wifi) hardware. There are a few around, so "drivers" for those would be needed.
This sets the baseline of features required. Multitasking will probably be unavoidable (clients need to be able to do stuff "in the background", like e.g. respond to IRC PING messages). There's already a LOT of complexity attached to that requirement. You'll need an executable format with relocation info, and a program loader doing the relocation on startup of a new process. You'll need a concept how to deal with the tiny hardware stack (partition it, relying on apps not to overflow? copy it around on each context switch?). You'll need a concept how to dynamically allocate memory (probably just page-wise). And so on.
A windowing system IMHO makes no sense at all on that machine. The UI should focus on text (maybe a set of virtual consoles?). Support for GUI apps might be possible, but then only "full-screen". 🤔