I don't think it makes good business sense to require more than the Core rules for the campaign setting book as you're substantially increasing the entry cost for new players to get involved. Instead, it makes more sense to include just enough flavour/inspiration in the setting guide to actually entice people to buy HFH to enhance their experience. So perhaps include some of the general rules changes that apply to the setting as a whole and provide plenty of sidebars and such that show how a particular rule/class from the HFH could be used in conjunction with what they already have. That way they can get going right away and add the HFH as needed/desired.
Sounds like near unanimity on this!
Just noticed this thread. I'll add my voice to the chorus: Auran Empire should use ACKS Core, for all the reasons mentioned (brand dilution, ease of use, playability, cost of entry, portability, etc., etc.). I think sidebars with reference to how the use of the HFH would adjust the setting would be acceptable, as long as they don't become too intrusive, but it's a fine line; at a certain point it's probably better as a standalone conversion document, either free, through Patreon, or on DTRPG if it's meaty enough.
Or an issue of Axiom...
Yes, that’s what I meant by Patreon.