I voted for 'Port the OS to new hardware', but I'd rather it were 'Reimplemented for new hardware'. What most users love about RISC OS is its essence; the way it works and feels. I don't think anyone really likes its foundations though.
I think Castle's gone some way to doing this already, but let's throw away the underlying decrepit mess, keep (and maybe modernise a little) the UI, and give it a nice pre-emptive multitasking x86 base to work on. Basically, what Apple did to the Mac between OS9 and X (though I realise that was a little more convoluted). With application-level emulation, existing applications could still run whilst new more advanced applications could be written/ported from other platforms.
In the meantime, ROX is a nice approximation to RISC OS on Linux, but still not enough like it for me to use it. Mac OS X has proved an admirable alternative for me =)