Taliesin Architects was founded to carry on the work and philosophy of Frank Lloyd Wright. And while I don't know if Wright ever actually designed a treehouse, I expect if he did, it would have looked a lot like the one his disciple John Rattenbury made in 1967 for an upscale family in Maplewood, New Jersey.
When he built the 9,000-sf Kessler House, Rattenbury threw in a treehouse for the kids, complete with hexagonal portholes. After finishing an extensive restoration of the house, Tarantino Studio redid the treehouse, too. It's awesome, though with that single support pole, it does look more like a birdhouse than a treehouse. To be perfectly frank.
Preservation | Kessler House [tarantinostudio.com]
Previously in sick modernist playhouses by proteges: Long Island playhouse by ex-Richard Meier architect Thomas Phifer