viewpoints
One of the moments I liked most in AC Unity was when you are trapped at the center of a hedge maze and have to climb a tower to see your way out. This moment plays against so many expectations that AC games drill into you: climbing the tower is not required to complete the mission, reaching the top does not automatically update your map with the maze layout, and there is precious little ceremony: you simply climb up and look out at the maze the way you look at anything in the game.
This moment is one of the few that embody a believable and natural implementation of a "view point" into Assassin's Creed's gameplay. Usually we climb things that are far too high to provide any realistic strategic advantage, and then we press a button to add the surroundings to our map as a big cutscene plays, and then this is completely decoupled from our actual experience of the game and the immediate concerns of whatever mission we're on.
I feel like viewpoints are given too much significance. They shouldn't be so obvious, they shouldn't be a design element that is so easy to point out. They should just be a natural part of playing a game where you're someone who can climb tall structures. They should be places where missions start, or perhaps just places the player seeks out for themselves. There should be reasons for the player to seek them out, but those reasons should be tactical rather than completionist.
Perhaps the increasing power of eagle vision is what's stopping this from happening: less need to get new angles on situations when you can see anything around you in any direction regardless of angles or obstructions.
I do seek out the viewpoints for tactical reasons (as well as completionist) now that they are also fast travel points.
I do agree with your premise.
“Force has no place where there is need of skill." Herodotus
This is only tangentially related but, I was playing Minecraft recently with a friend, and she'd built a Dock in the distance. From our house, looking in the direction she built the dock was useless, for in my sightline was a massive forest of trees.
I built our house higher, into a tall tower. From the top of it, I could see the Landscape better. I knew which direction I needed to chop Trees in, in order to clear out forest so we could build a proper road to her Dock. Problem was, by the time I climbed all the way down the stairs, I'd kind of forget which direction I needed to move in, so I had my friend dig out a 3-Block deep pit of water at the bottom of our tower - the minimum required to survive a fall into it - and I'd just Leap into it from my viewpoint.
This felt incredibly rewarding, since I now knew which direction I needed to chop in, I had a way of seeing the "big picture" again after having changed the game world around me it was all inherently useful to my grand goal.
The two of us had inadvertently created a somewhat Assassin's Creed-ish experience totally outside the confines of an Assassin's Creed game, using the aesthetic principles the AC games would suggest in a more plausible and practical way.
It would be absolutely awesome if Assassin's Creed learned from that feeling, or at the very least, that set of steps. The moment you bring up from Unity is a really great one, and one I hadn't thought of at all until now. I doubt many players have, but you're striking something big in talking about it, since it's basically part of what Assassin's Creed is "about."
I do think there's something to consider in McStab's comment. Viewpoints now ARE Fast Travel points, and Fast Travel is something that, with the ludicrous size of these new cities, shouldn't be gutted completely. I propose an idea I first heard about in a game design video by Mark Brown, talking about the Checkpointing System in Ori and the Blind Forest. In that game, the Player decides where to put down Save Points or Checkpoints, which they can do at any time. Doing this drains an orb of their Energy, which they could have used for attacks or other actions.
To merge the more game-y nature of Fast Travel points with Jermaine's excellent idea for the more organic use of Viewpoints, what I'd suggest is that a player can set their own Fast Travel points anytime and anywhere they wish, but it would have some kind of cost or requirement, and if a player sets more than some arbitrary number (say, 10 of them), the very first one they placed would disappear to make room for the eleventh one, for example. The Animus can just do that, y'know? Offer a Subject the ability to place Anchor points in the Simulation Space.
Meshing these ideas together would work, I believe, because there is no expectation that a player will find something game-y, or some kind of Icon or Activity at the top of each tall structure they scale, but they certainly CAN set an Anchor Point to them if they WANT TO - which is entirely up to them and their decision.
As for unfogging the map, I think I really would prefer if that were done more fluidly, and without having to press a button to Sync, or playing a Cutscene. Get to the top of a tall structure, and look out over the horizon until High-Ground Icons trigger. Their triggering is effectively the cue that would tell the player that this area has been Synced, and they can then continue their gameplay-flow without interruption. For that matter, it's not much of a mechanic to press a button to unfog your map. It'd be cooler if we actually had to look around at the environment ourselves. Then, rather than having icons marking the next viewpoints or tall towers, the way the player could puzzle out which one comes next is by looking at their map (let's have it be an actual item in the character's hands) and seeing which areas of it are not filled in, or not drawn-in. They could then use logic and reason to figure out, "Aha, there must be a tall structure I haven't yet climbed in that area, but which one it is exactly is something I'll just have to go discover."
I completely agree with that idea.
How about to ease the player into it, there are 2-3 built in fast travel viewpoints (maybe one near the hideout - if it's not a moving train). Then let them set 10 others. Also let them remove them at will too though, so that if the first one they set is more important to them than the 5th, they can remove the 5th first and have it not count against the 10 allowed.
I imagine this would open up a whole can of worms for speedrunning too. If fed or phi want to chime in... 
“Force has no place where there is need of skill." Herodotus
Sure. The only caveat would be that to set a Fast Travel point, you can only do it where you're currently standing. This makes sense and is fair, because your body has to have been able to reach it. I'd suggest also eventually removing the first three, so as to not have anything be set in stone. Rebecca and Shaun can offer those first three to the player to start out with, but once they're comfortable enough with setting their own, they can set their own Animus Anchor Points as they wish. If they really want to keep those first three, they can, but having them be part of the 10-point limit (therefore, only seven more can be set unless they remove those first three at some point) it solidifies the player's need to grow out of that starting phase.
"Abstergo's been running interference ever since they caught wind of us using Initiates for datamining. Right now, they haven't done much damage, but it's probably going to make your life a little harder. We can't quite break through all the walls necessary in order to code in all of this simulation's anchor-points, but we've managed to get you three. The rest, you're just going to have to do yourself. Good luck in there."
[Synchronizes]
hmm I like the idea of fast travel as a resource (as long as its set up in a way that clearly keeps microtransactions off the table) Could give restricted zones a more multifaceted purpose: make them places you can't fast-travel into.
And @DoubleStab, I disagree that fast travelling is a tactical choice. You can't fast travel during missions which is the only time you could potentially use it for a time-sensitive task, and outside of missions nothing in the game is time-sensitive. they currently only really exist to mitigate busywork, which is arguably valuable but not at all tactical.
I think DoubleStab may have meant it like that, and the phrasing just got in the way.
About keeping microtransactions off the table, yes please. This would be a big part of a player's navigation, and could even make the Animus an active entity that exists mechanically, instead of just being a filter through which the player sees the simulation. I'd rather not have its balance thrown off or skewed in favor of "fixing" it by injecting $$$.
I love the idea of view points becoming more natural. I'd love to be able to climb a random, unmarked, high building and look around for places of interest. I'd even go so far as to not have any icons pop up once you're on that high spot. What works for me is that the player has to actually use his own senses to look around, then inspect places that look interesting. Zoom-in and hold sight on a market or a courtyard from your vantage point to scan it, revealing it on the map and maybe giving some database info. Collectibles can be scanned in eagle vision.
Not sure how I feel about setting your own fast-travel points. I don't really like the way viewpoints are fast-travel locations. It doesn't make a lot of sense to me. Say you're skipping through time, travelling from A to B. Why would B be on top of a tower? I liked how it was done in AC2, Brotherhood, Revelations and AC3 , with carriage stops or underground tunnels being the fast travel points. I guess letting the player drop anchors puts more responsibility and choice in the player's hands, and I'm all for that. I prefer if they make it 'logical' from an in-timeperiod point of view (tunnels can be explained as being the mode of transportation for resistance groups), but I guess they could explain player-set anchors with the modern-day mechanics and Animus technology (DAZ's comment).
Agreed that my language got in the way. What I meant was only that I use viewpoints for reasons other than synchronizing all the viewpoints.
“Force has no place where there is need of skill." Herodotus