A sinusoidal temperament

Here's a temperament where the size of the fifth varies sinusoidally
around 700 cents; four of the fifths make a pure 5, so we have one
pure 5/4 in this system, and another twice as sharp as the 400 cent
major third.

It's a rather extreme temperament ordinaire, maybe what Margo would
call an extraordinaire. In general I think bifrost is a better choice,
where the very sharp thirds become 14/11s.