Just all optional caves, dungeons and whatnot. Otherwise, what’s the point for their existence other than sheer curiosity if you’ve never seen it before? Just balance out the monster density, difficulty and item drops per area to make sure they’re also actually viable farming spots compared to the current meta for level 85 farming spots.
In 2.4 PTR, some of the new level 85 areas like all the Act V portals have such low monster density they’re just not worth farming. Especially as you have to find them first and the monsters could be more dangerous for some builds wearing MF gear. Then you have some areas like the Arachnid Lair, which would also be a level 85 area, right next to a waypoint, with better monster density and easier monsters to farm with just MF gear. More level 85 areas would increase variety in case people get bored, but eventually they would still be used far less than the more convenient ones in that case.
If a place has lower monster density, either increase the density or improve the drop chances. If they are notoriously difficult (level 85 areas with gloams/dolls/other enemies that are difficult for some builds), make them worth it compared to easier level 85 areas.
It’d probably also be useful if they’d look at equal opportunity for farming spots considering monster immunities and then make those spots equally rewarding (difficult areas or lower density areas all in all giving the same drop chances/rewards per area clearing). Otherwise every build will just have a form of “best in slot” farming area and 95% of people will just run that one repetitively instead of looking at the others.
Right now there’s quite some people who just get burnt out from doing the same run thousands of times, but then also feel like they’re wasting time if they go to other spots for variety knowing that they’ll be less rewarding.