[Skarg]

I think ITL already provides that in the form of the travel rules, though they are a bit scattered in different places, and leave it up to the GM to do the rate/time/distance math for situations other than "how far can we travel in one type of terrain in one day?". A well-done improved set of travel & exploration rules is something I'd personally love, and would be useful for this sort of product, though I'm not sure framing it in terms of "hexcrawl rules" rather than "travel & exploration rules" would be helpful for other than some specific format of adventure content.