Pondering what might make a good par-6---good enough to warrant having one. And trying to think of what par-5 I know, that could be extended to a par-6.
It seems to me that neither sheer distance, nor the number of doglegs, would do it. The ratio of long throws to putts is just too great. And it seems to be to be too much of the same thing.
(Disclaimer: At my current point of deterioration, some holes are already essentially par-6s for me. And they're not all that much fun.)
But if you had a landscape with enough different features---with a lake to throw over, maybe an island to land on and then throw from, a hill to throw down, a hill to throw up, or a hilltop-to-hilltop throw, or something else---you might be able to cobble together a par-6 that made a variety of demands on the players, enough to justify it.
Then, of course, you'd have to have terrain for 17 other great holes, to avoid thinking that this one par-6 should have been split up into two good holes, instead of using all that great stuff at once.