1. There's definitely big value in adding diverse activities, but I think devs have to be careful that the activities are actually replayable without feeling "oh it's this minigame again."
In WoW, they add a lot of minigame type world quests (they're objectives that spawn across the expansion's entire open world and have 24 hr respawn cycles or smaller, some 2 or 3 hrs). These include puzzles, timed activities, kill x y amount of times, fetch quests, kill rare spawns, etc.
Although I think most people enjoy the varied, new content like puzzles and timed activities at first, they generally avoid them within a couple weeks of a new expansion or patch introducing it, and general chat turns into "ugh this thing again."
Longterm, players always end up maximizing whatever is the quickest and simplest end to efficiently get whatever valuable thing is rewarded. In WoW's case, people basically just do the quick and easy rare spawn kills, then the kill x of y monster quests, then the fetch quests or puzzles, depending on which is quicker.
2. The rewards have to be able to justify the time/effort investment without the content feeling mandatory because the rewards are too good. I'm not informed enough on D4's character progression to make any points here, but I think it's important to think about.
3. You make a good point here, and I think this is worth discussing:
(after blizzconline 2021 update) - i felt this idea would make even more sense after the information displayed in this blizzcon, since this encampments will be improvable, you could make them improve faster with and this way opening new townportals or crafting recipes. one think thought, these encamptments should not be permanent, haveing somekin of event that can and will probably destroy them or at least weaken them (a demon or heaven invasion, so they will never stay the same adding even more randomness in the game. This ideai i think is the better of the 2 because it makes so much sense and the part of mount equipment and dismounting on attacks is already on their plans.
Any world upgrade rewards (like camps that you mention) add the appeal of immersion and visual progression. One of my criticisms of D3's endgame systems is that most of them you just jump in a portal into some random realm and do a thing. It made the game (to me) feel less grounded, more fantastic, and not as rooted as the first two.
I like this idea you bring up of camps being able to degrade, but the camps would have to have a function or value in terms of character progression to make them worth sustaining in an area. Like, when they're maintained, a passive buff to drops in the surrounding overworld area and dungeons, or a buff to xp (ONLY if they're doing something like paragon leveling again--otherwise XP buffs would be useless for endgame), or a passive reduction in certain crafting types while at the camp. (I'll admit here I haven't really kept up with news so this might already be a thing.)
The lack of permanence is also critical like you point out--if you can just upgrade them all the way and they never degrade, it sort of just becomes an assumed endgame system. Like upgrading vendors in D3--once you do it all on your main (and it doesn't take long) you just never think about it again and most of the vendors don't really have much additional depth beyond itemization quirks.
I don't really know anything about the ancestral system, so I can't comment there.