> BTW, shouldn't `sensibly' be the default?
  > Or even the only acceptable setting (besides setting it to
  > a home-cooked function)?
  > I mean the nil value can be simulated with
  >
  >    (setq split-width-threshold most-positive-fixnum)
  >
  > and the `horizontal' value can be emulated with
  >
  >    (setq split-height-threshold most-positive-fixnum)
  >
  > If most-positive-fixnum is too inconvenient, we could accept a nil
  > setting for the thresholds to mean the same as most-positive-fixnum.

Is the attached patch what you mean?  There's yet no clever default for
handling the height/width aspect ratio.