So, I think we need to start looking at ways to make content without a grind. Not saying grind is good or bad, but all kinds of content is always good.
I think that the most extreme, but maybe most amazing thing that could be implemented would be a near-impossible dungeon/primal/boss, or you know what, even a super hard jumping puzzle, (Devs said they would look into making jumping puzzles) essentially content so hard that even the devs can barely beat it.
For the boss content to be truly hard, it can't be scripted, otherwise people will just guide it up and that's that. It would have to be a fight where you can only survive through reaction timing only, I suppose Dark Souls could be a good example, but who knows.
The jumping puzzle content would be something that could only be found through sheer luck and determination, after all we need to keep in mind this content is supposed to last a while without being a grind. I think a good way to do this is to randomly spawn an entrance somewhere around the map to the instanced jumping puzzle, and depending on where you found the entrance, whether it's leading into a mountain, or through a waterfall, it would lead you to a different jumping puzzle, it would be the devs choice as to whether or not these entrances would randomly appear, but again, I don't think making it too scripted is a good idea.
Now for all this near-impossible content, it should of course be for some legendary reward that has some good lore behind it, what the item(s) could be is up to them. Probably not weapons, since we have the Relic upgrades going, but maybe Relic gear...?
On a side note: We've got a lot of vertical progression now, I'm hoping we'll get other forms of progression in the future!
P.S Ideas like these would most likely require new mechanics and systems, so keep an open mind, aka I didn't put much thought into it, and I don't think it should be completely random, unless it could become a good Idea.