I was looking back over old material and notice that I did an "Improv Underworld Part I" seven months ago, but never got around to doing Part II. I may have to stare very long at the old improv underworld posts to figure out what I was planning for Part II.
I discovered the omission because I was looking specifically at my Last Minute GM posts on the improv mapping topic. After all, I've done a lot of stuff already that could be relevant to an improv hexcrawl; I just haven't finished and integrated the rough suggestions.
I figure two techniques are going to be important. One is the trick of using reaction rolls for the "reaction" of abstract concepts or resources to the characters. If the next hex is a Hostile wilderness, Food or Water may be scarce, or the beasts may be deadly, or all three.
The other trick would be to adapt the impromptu routes trick to other kinds of path-like features. The original post is about routes to other settlements in a given direction: basically, when plays ask "what's north of this village?" you roll 6d6 and read the results from left to right as the next six settlements to the north, in order, with the numeric results as the number of leagues in that leg of the journey (multiplied by a modifier for lightly populated, sparsely populated, or true wilderness areas.)
I did do something similar in one of the subhexcrawl posts to cover roads and rivers, but I may have introduced some unnecessary complications there that wouldn't be good for a true improv hexcrawl. I'm also thinking how to adapt the simpler form of the technique to biome changes.