Some solid, at least nodding toward simulation/realism survival rules (movement, foraging, getting lost, etc...) built on the 5E math but can be new since 5E itself really doesn't do this part very well, or at all.Can you tell us what your ideal hexcrawl ruleset would include?
Interesting, internally consistent and potentially deep hex populating tables. Lots of non 5E sources will have this, but the benefit of the book being made for 5E will mean any mechanically meaningful results won't have to be converted.
Less important: a smart random terrain system. This doesn't actually need to be related to 5E in any meaningful way, and there are many online solutions, so it isn't strictly necessary, just mentioning for completeness. I would rather generate my sandbox randomly and then tweak it to what I want than to make it up myself (since I generally prefer random seeds in worldbuilding overall).