I noticed during the 3.X series heavensward seemed to get alot of love with granting a hm dungeon to a hw dungeon. While ARR was left out

ARR: Hardmodes possibly
Arurm vale
1000 maws of toto rak
Dezmael darkhold
Cutters cry
snowcloak
Keeper of the lake
It would be nice to revisit these and see what time has done to them since we cleared them

As for possible hw hardmodes
Saint Mociannes arboretum
The aery
Neverreap