Part of the reason I mentioned "editor/compiler/debugger ecosystem" combo is that I know there are people on HMN already working on each of those things, so the expertise is already there to some extent. But of course there's no reason why it necessarily has to be that, or why there can't be more than one thing, as you mentioned.
Yeah we definitely want a great ecosystem for developing for the platform to attract developers to it but I think we want additional killer applications outside of that to highlight that the simplicity provides benefits to general users, not just developers.
When I think about things where platform/OS complexity is the major limiting factor in productivity, audio is actually a big one that comes to mind. Between different audio cards, different audio devices, different audio sources, different audio formats (sample rates, bit depths), different audio this that and the other thing, it seems like there's always a number of problems that come up and require a number of workarounds. For example, FL Studio ran dog slow on my old win7 box without third party drivers, which were themselves unreliable. Recording system audio on macos requires third-party drivers and a lot of fiddling with settings. And recording multiple audio sources simultaneously to different tracks is a simple and extremely useful feature that doesn't seem to be well-supported anywhere. So I think that could be a topic worth looking into.
Audio is a great suggestion; I think that would be a good area to try to shine in!
Another one that I'm thinking would be good would be security - a simpler system has less attack surface and thus we should be able to create a more secure system. The hypervisor thing plays into this well too in that if you make the hardware so simple to program directly that you don't really need a full blown OS the hypervisor gives you multitasking but it also gives you security through isolation. That is the selling point of Qubes OS. I keep bringing up Qubes OS as an exemplar but I should clarify that it is not an exemplar of simplicity and good quality, it is just the closest thing I am aware of regarding a hypervisor-based desktop. If you have a spare laptop I recommend trying it to get an idea of what I'm talking about with regards to a hypervisor based system. I used it as my sole OS on my laptop for a while and while it was not perfect it did work well enough that the only thing that made me switch back was that I wanted to do screen sharing with my gf sometimes and it doesn't allow that by design (you can work around that various ways with various trade-offs if you really want to but I just switched back to Arch Linux). On my laptop I would maybe have like 5 or 6 Linux instances going at once and it handled it fine, applications in different VMs running seamlessly side by side. Imagine that but instead of full blown Linux instances with multiple applications running in each, you've just got a bunch of bare metal applications all running seamlessly side by side (probably most of them using some optional standardised minimal OS layer provided by the company, think exokernels, think unikernels like IncludeOS)