When I think of doing something like this, I default back to Tupp's yearning to repurpose, and reuse, which fits the spirit of these types of projects for me.
Yurts
She Sheds
Sea Container builds.... they're all interesting, economical choices, IMO.
And what about little trailers that can just GO when you want to go?!? WIth hook ups for water, electricity, etc? We had friends who treated their camper as a little guest house.... and it was sitting unceremoniously in their drive way. Ny Great Grandparents kept a little trailer on my Grandparent's land, for visits. There are so many options, and ideas. I think it's fun to dream, and no two people will have the same ideas about what's best or most appropriate.
If you guys find really cool ideas, just share them on this thread so we can find them easily.
Lighter